Does all reach 'threaten' in 2024?

Under the Reach entry in the PHB (pg. 214) it says the weapon property Reach permits OA's beyond 5 feet. 

The Sage Advice Compendium backs that up:
https://www.dndbeyond.com/sources/dnd/sae/sage-advice-compendium#SAC-Combat16

What does this mean for monsters? Do all monsters with reach have what previous versions considered Threatening Reach?

Quote from GaiusMarcus >>

What does this mean for monsters? 

The general rules for this are the same for all creatures, unless a specific creature has a specific exception to it. If an enemy leaves a creature's reach, the creature can make an opportunity attack against them as a reaction.

@GaiusMarcus, this thread might be useful for you: Reach and Opportunity Attacks?

From it:

Quote from Tarod_ >>

There are also some conversations with the Dev about this topic. Let me use quotes from another thread:

Quote from Plaguescarred >>

You can have multiple reach, for example many monsters do as well as characters doing Opportunity Attacks with reach weapon.

This subject has been clarified by Devs in Sage Advice Compendium and on Twitter in the past.

How does a reach weapon work with opportunity attacks?

An opportunity attack is normally triggered when a creature you can see moves beyond your reach. If you want to make an opportunity attack with a reach weapon, such as a glaive or a halberd, you can do so when a creature leaves the reach you have with that weapon. For example, if you’re wielding a halberd, a creature that is right next to you could move 5 feet away without triggering an opportunity attack. If that creature tries to move an additional 5 feet—beyond your 10-foot reach—the creature then triggers an opportunity attack.

@samiam8910 If a monster has 2 different reaches, when does it get opportunity attacks? Like the roc’s talons at 5 feet and beak at 10.

@JeremyECrawford If you have more than one reach, a foe provokes an opportunity attack when it leaves any of them. #DnD 

@samiam8910 Isn't this directly against your official SA article from July 2015?

@JeremyECrawford The SA answer is only about making an opportunity attack with the reach weapon, not with another reach you might have.

@samiam8910 So the book is incorrect in saying OAs are for when leaving your reach when it should be your weapons' reach

@JeremyECrawford "Your reach" means whatever reach of yours is relevant at the moment.

 

Quote from TarodNet >

 I'd also like to add the following conversation (link to sageadvice.eu):

@jd2319 What if I'm dual-wielding a dagger and a whip, can I use the dagger when someone leaves my 5' but stays w/in 10'?
@JeremyECrawford If you want to make an opportunity attack with X, the attack is triggered when a foe leaves the reach of X. X = weapon you want to use
@thethain Awesome! So does that mean you can always make a 5ft unarmed Opportunity Attack, IE if holding a two handed weapon?
@JeremyECrawford Unless a feature tells you otherwise, you have a 5 ft. reach with your unarmed strikes, and you can make opportunity attacks with them.

Or this example:

Quote from Plaguescarred >>

Quote from Character77006 >> 

If someone moves 30" then there is no opportunity attack?   That is what you just said, by moving beyond 10 feet results the bite can't be used. "..****y provoke an Opportunity Attack with the weapon or action with corresponding reach"

 I'm not sure to understand if by 30'' you mean 30 feet or 30 inche, which double apostrophe is typically used for.

Moving occur foot by foot or squares by squares in Grid play. So if for example a creature adjacent to a Otyugh moves away 30 feet, it will provoke Opportunity Attack when moving beyond;

05 feet or 1 squares =  Bite reach 5 feet 

10 feet or 2 squares = Tentacle reach 10 feet

15 feet or 3 squares = N/A

20 feet or 4 squares = N/A

25 feet or 5 squares = N/A

30 feet or 6 squares = N/A

Quote from GaiusMarcus >>

Under the Reach entry in the PHB (pg. 214) it says the weapon property Reach permits OA's beyond 5 feet. 

The Sage Advice Compendium backs that up:
https://www.dndbeyond.com/sources/dnd/sae/sage-advice-compendium#SAC-Combat16

What does this mean for monsters? Do all monsters with reach have what previous versions considered Threatening Reach?

Terms such as "threaten" or "threatening" as related to distance between creatures don't really have any mechanical significance in 5e or 5.5e as they might have in previous versions.

As for any rules which might or might not interact with the reach of a creature, such rules would have to be evaluated on a case-by-case basis.

As you've pointed out, the "Reach" property of a weapon does interact with a creature's natural reach as follows:

Reach

A Reach weapon adds 5 feet to your reach when you attack with it, as well as when determining your reach for Opportunity Attacks with it.

To properly calculate this, you would need to know what "your reach" was in the first place.  By default, from the Rules Glossary, we apply this rule:

Reach

A creature has a reach of 5 feet unless a rule says otherwise.

Here are a few perhaps counterintuitive examples where we might expect a certain rule to interact with a creature's reach, but it does not:

-- Ranged Attacks:

Ranged Attacks in Close Combat

Aiming a ranged attack is more difficult when a foe is next to you. When you make a ranged attack roll with a weapon, a spell, or some other means, you have Disadvantage on the roll if you are within 5 feet of an enemy who can see you and doesn’t have the Incapacitated condition.

This means that if an enemy is 10 feet away from you and that enemy has a reach of 10 feet, your Ranged Attack is nevertheless unaffected.  Likewise, it also means that if an enemy is 10 feet away from you and your reach is 10 feet for some reason, your Ranged Attack is still nevertheless unaffected.

-- Attacking an Unconscious Creature:

Automatic Critical Hits. Any attack roll that hits you is a Critical Hit if the attacker is within 5 feet of you.

So, if you are standing 10 feet away from an Unconscious creature and you have a reach of 10 feet, your attack roll is NOT an automatic Critical Hit.

-- Unarmed Strikes:  (This rule probably needs errata, but that hasn't happened yet)

Unarmed Strike

Instead of using a weapon to make a melee attack, you can use a punch, kick, headbutt, or similar forceful blow. In game terms, this is an Unarmed Strike—a melee attack that involves you using your body to damage, grapple, or shove a target within 5 feet of you.

This means, as written, that a monster with a reach of 10 feet that is standing 10 feet away from you cannot attempt to grapple you unless their stat block explicitly says otherwise, for example.  A DM is likely to house rule this one in such a way that things make more sense.

The overall point is that the text of a rule must be examined to determine whether and how that rule might interact with a creature's reach.