Just wanted to come in here and back up my boy HAM real quick. Criminally underrated feat. You do not need to chase cutesy things like Armor of Agathys synergies to feel like a boss with Heavy Armor Master. Picked it up at level 4 on my Centaur Ancients Paladin and it's still putting in crazy work at level 7. 

Quote from sfPanzer >>

Eldritch Armor was a nice idea and definitely NOT too strong or anything, but having the option to multiclass made it kind of obsolete.

But that's actually why it's kind of a weird omission; in Tasha's Cauldron we got a bunch of feats that made it easier to get some of the benefits of multiclassing without losing progression, and over the years we've had various sub-classes that give us a bit of something from another class so we can play things without the compromises that multiclassing requires.

While a quick dip into Fighter will absolutely work well, it's not free; you lose out on your Warlock spell progression and higher level features. Eldritch Armour might not have been quite as powerful as a Fighter dip, but it also would have only cost an Eldritch Invocation choice, all other progression would remain the same. Plus with the ability to change your Eldritch Invocations at later levels, you could have taken Eldritch Armour early, then taken the Fighter dip later and swapping invocations, so you'd avoid having to use light armour or Armor of Shadows as a stop gap.

Not that there's anything wrong with Armor of Shadows as a stop-gap, but thematically it's not the same as being able to actually just wear armour, and of course it doesn't let you wear shiny magic medium armours long term.

Quote from sfPanzer >>

A dip into Fighter is not free but it's just for one level if you just want the armor proficiency without the action surge. You lose practically nothing. Just the chance to get the Eldritch Master feature nobody really ever cared for anyway. The cost for what you get is laughable low. I'd even go so far and say that the Invocation slot for Eldritch Armor is more valuable than that one level especially when considering all the things you get.

Loss of progression isn't a "laughably low" cost; I think it's too easy to focus on what you're gaining and ignore what you'e losing. Being one level behind means you gain access to new spells later, your ability score increases come later, you get additional invocations later etc., people will often argue that these are only delayed but you only get a finite number of levels; the more you delay spell progression, the less of the campaign you have to access to those spells, which matters on a caster, including the warlock.

Now obviously whether the trade off is worth it depends on what exactly you're trying to build; if you're all about building a martial fighter/caster then by all means grab the level(s) in Fighter, but having an invocation would have meant we could remain pure warlock and still pick up the proficiency without any compromises. It's also worth remembering that multiclassing is an optional feature, eldritch invocations are a standard part of a Warlock's toolkit.

The real question for me is wether the added benefit of Eldritch Armor (the rapid don/doff time) is enough to balance it against other invocations; my gut feeling is that it's not quite enough, as the quick don/doff is way too situational a thing to actually use. It also feels a little strange, as I feel like warlock gear should imply more of a permanent bond. I'd prefer to see it be more Armorer-like (must spend time designating a piece of light or medium armour as your Eldritch Armor) then maybe have a more useful added benefit, for example; whenever you roll a saving throw against being restrained, stunned or knocked prone you can roll one of your hit die and add it to the result?

Quote from sfPanzer >>

A one level delay really is nothing for a Warlock. I feel like you are greatly exaggerating what you're losing by doing a one level dip here. Not wasting an Invocation slot on armor is worth much much more.

You're doing literally the opposite; pretending there's zero cost doesn't make it true. Like I said, it's going to depend on what exactly you're aiming for with a build; there are trade offs both ways, or there would be if there was another way, which was supposed to be the entire point of the thread.

But whatever, I'm not going in circles on this.

I never allow multi-classing in my games and the fact it is an optional rule should be considered.

Let's be honest, how often does the time necessary to put an armor come into play? In 10 years of game, it must have relevant about two or three times at my tables. The real advantage it gave wasn't doning or taking off the armor in a single action, it was the free proficiency in medium/heavy armor, which is downright necessary to make Pact of the Blade useful for anyone other than Hexblade.

As far as I'm concerned, Eldritch Armor is authorized at my table.

Warlocks have existed as long as devils, djinn, celestials etc have. It would be nigh impossible to have the setting "long before Warlocks"

Quote from HeironymusZot >>

You gain the following benefits while wearing this armor:

• If the armor normally has a Strength requirement, the arcane armor lacks this requirement for you.
• You can use the arcane armor as a spellcasting focus for your artificer spells.
• The armor attaches to you and can’t be removed against your will. It also expands to cover your entire body, although you can retract or deploy the helmet as a bonus action. The armor replaces any missing limbs, functioning identically to a limb it replaces.
• You can doff or don the armor as an action. 

This is clearly just a way to inject Iron Man into your Artificier class - or perhaps Doctor Von Doom, depending on our alignment.