I'm curious if i'm the only one who does this sort of thing?  And if not, what are some of the rules you implement to depower your players.

i started weakening the long rests. that has worked a lot on players.

basically i use the variants in the DMG.
long rest no longer fully heal you. you still requires the use of hit dice and so if you did use them during short rest. then you only get back half of them during long rest. which you cna use again right away. but thats if you don't want short rest later on. just that alone changes everything. i think thats the only real change you need.

but for flavor and making them be usefull... i added the medical kit dependency where you need to have a healing kit to actually heal.

just that is enough for them to think as hit points as ressources that can be depleted now.
of course theyhave healers and thus if remaining spell slots are left, they use that.
but still this made it a good starting point.

I don't see the point when I can stay within the rules and just strengthen literally everything else.

Quote from scatterbraind >>

I don't see the point when I can stay within the rules and just strengthen literally everything else.

 then how do you deal with players trying to long rest every fights ? aside from trying to make them fight every minutes ?
thats the problem, the game is too easy for players still. yes monsters can be made stronger, but isn't it against the engine itself to just make monsters stronger ?
at base, fights in the game, except at low levels... the game is easy to survive and in the end, it becomes easier and easier for players to survive. Even monsters 10 CR above them.
as an exemple of a beholder who got 1 shotted by my players in 1 single round. he was 8 CR above them. yet they did over 300 damage in one single turn. and even his 5 blast couldn't down them. they were level 10, the beholder was CR 16, boosted to CR 18. they had no magical items, they just had literally what their class allowed them.

power creep is a thing still... much less then previous editions, sure... but still much power creep as the game goes. up to level 7-8 it works as if... above that, players start to creep up the power levels and fast.

RAW, I believe, dictates that you can only long rest once in a 24 hour period. I enforce this with a little flexibility, but I enforce it.

I don't worry about it too much personally, but in an upcoming game I might, because of #Reasons. What I do do, however, is put time limits on things when it makes sense, and I always keep the plot moving forward with or without the players. So if they decide to prioritize a long rest over saving a village from bandits, now they have to liberate the village of bandits instead, and they won't have any way of resting at all during that time.

I force the story forward without railroading by having events happen quickly, but not too often. It's called the One-Two-Punch.

Mechanically, however... Disallow feats and flanking, which are already optional, enforce carry and climb weight rules strictly, milestone level instead of exp to control the pacing, make identify a skill check roll with higher dcs for rarer items (and don't allow that nonsense "you can identify a magic item over a short rest" rule), require ammunition to be tracked accurately, require spellcasters to track all their material components (you don't suddenly have a component just because you leveled up and took the spell), don't gold to be used in replacement of actually acquiring gold cost material components, enforce stricter survival rules, or gritty survival...

Some of these things may make the game less fun for your players, so always discuss the ideas first and ensure your players won't hate you for the new rules for ruining their fun. 

Even with the fact that you only do get one lro per day unless otherwise mentioned - why aren't they being attacked mid-rest? That would surely be the moment that would have them be the most vulnerable - but imo, if you are DMing just to kill the players, that's already a loss on your side.

Don't use combat generators nor stuff of that kind, as every party is different by itself: A party made of martials with no magic weapons will have issues overcoming a guy with resistance or immunity to non-magical weapons, but I haven't seen yet a party with no sort of caster, so think about actually giving them a contest if free combats is what worries you, not by CR, but by threat related towards the state of the party.

Also if they require a long rest after every combat, do note another thing: You have exhausted all or almost all of their resources, to a point short rests (for Arcane Recovery, healing or some feature recov.) won't work as you will summon another monster that will kill them if they ain't at 100%.

There is also a lack of warlocks, fighters and such, I assume.

Here's what matters - Is your party having fun in the encounters that you are building? If they are, great job! If they respond with "more challenge please", I might suggest turning up the "ruthless" dial on your monsters. Your players should have a 70% chance of winning and a 30% chance of failure, but they should feel like they have a 30% chance of winning and a 70% chance of failure.

I'm on the Nope Train when it comes to the idea of weakening PCs after a point that the DM realizes that their party is powerful. If your intent is to make your world gritty, then start it out that way. I'm all for challenging and meaningful encounters in worlds where "life is cheap and death is an everyday occurrence to the unwary".

How the DM designs and executes the encounters has a lot more to do with survivability than does the CR of any single foe. Use the environment against them, that's how battles and wars are won. In the stated example, a beholder, in its lair, should have the home field advantage. The beholder would have built their lair to prevent party shenanigans. Beholders, in particular, would want some minion types around to aid in shenanigan prevention strategies in said lair. 

If the party is allowed to be fully rested just prior to your BBE encounter, they will always hit above their weight class. The mention of rests has already been touched on, specifically the "only benefit from one Long Rest per 24 Hours" part. Even so, if the party leaves the "dungeon" to rest, the enemies get to rest as well... PCs take a long rest, so do the enemies. They live here, you've invaded their home, and you're camping in their home office/guest bedroom and you expect nothing to happen? No reinforcements? No shift in their battle plan?

You don't have to homebrew anything new to weaken them, just use the tools you're given to wear them down before the fight. Use smart, ruthless, bloodthirsty enemies. You'll have the TPK you're looking for in no time! If you work it right you can even do it with Kobolds, Tucker's Kobolds! This package also comes in a lovely Orc color.

instead of telling players how they should play their games, you should try to help them achieve what they are asking for.
so right now people... stop trying to tell them that they shouldn'T weaken their players cause thats how you would play it. they are not you.

as for enforcing the 24 hour long rest... i tryed that often, but the problem is... its not realistic at all.
unless you consider that you as a person cannot sleep more then 8 hours a day and cannot be forced to just go back to bed whenever you want.
if your polayers want to take a long rest during the dsay and wake up during the night, they should be allowed to do so. otherwise its unrealistic.

fighting players during the rest to challenge them...
you do realise there is no ruling against them just fighting all night and still get their long rest right ?
they literally need to sleep only 6 hours a day to get the long rest and the last 2 hours can be anything that is not big activities... so you are saying that to interrupt a long rest you actually need to make them fight for like 3 hours in a row. good luck trying to challenge them during a night raid. oh right... they are the weakest right ? sleeping in armor is not a ruling at all in the game. and thats picking your players if you decide to always fight them during night time cause of no armor.

overall... all i see right now, are people who just want him to play Their game. thats not what he's asking for.
he's asking for help on a matter that requires him to balance a game toward easier player death.
not how to improove encounters by pumping more enemies.

i'm with him on this side... i challenge my players often... making new rules is the way to go... cause more monsters or bumping up monsters stats doesn'T really work without making power balance issues.

Quote from DnDPaladin>>

instead of telling players how they should play their games, you should try to help them achieve what they are asking for.
so right now people... stop trying to tell them that they shouldn'T weaken their players cause thats how you would play it. they are not you.

as for enforcing the 24 hour long rest... i tryed that often, but the problem is... its not realistic at all.
unless you consider that you as a person cannot sleep more then 8 hours a day and cannot be forced to just go back to bed whenever you want.
if your polayers want to take a long rest during the dsay and wake up during the night, they should be allowed to do so. otherwise its unrealistic.

This is an optional rule, much like the rest variants that have already been suggested. Your players can sleep in armor, that's true. I'm just trying to point out that there are rules that are provided for DMs to use that have been vetted through WOTC for use in the game.

overall... all i see right now, are people who just want him to play Their game. thats not what he's asking for.
he's asking for help on a matter that requires him to balance a game toward easier player death.
not how to improove encounters by pumping more enemies.

i'm with him on this side... i challenge my players often... making new rules is the way to go... cause more monsters or bumping up monsters stats doesn'T really work without making power balance issues.