Just noticed while trying to make a multiclassed warlock that some Eldritch Invocations have the prequisite "5th level warlock," and others just say "5th level." This should mean invocations like One with Shadows or Thirsting Blade are usable on 2nd, 3rd, or 4th level warlocks who have a total level of 5. However, it appears these invocations are coded as if this were not the case. I would appreciate if this could be changed, as I was looking forward to playing a character concept I had in mind, which is made impossible by this.

Eldritch Invocations

In your study of occult lore, you have unearthed eldritch invocations, fragments of forbidden knowledge that imbue you with an abiding magical ability.

At 2nd level, you gain two eldritch invocations of your choice. Your invocation options are detailed at the end of the class description. When you gain certain warlock levels, you gain additional invocations of your choice, as shown in the Invocations Known column of the Warlock table.

Additionally, when you gain a level in this class, you can choose one of the invocations you know and replace it with another invocation that you could learn at that level.

If an eldritch invocation has prerequisites, you must meet them to learn it. You can learn the invocation at the same time that you meet its prerequisites. A level prerequisite refers to your level in this class.

Invocations are based on Warlock level, and/or require prerequisites to be met (if you haven't already), as indicated in bold under the description for Eldritch Invocations. To my knowledge there aren't any multiclass exceptions to this, but I do see how one may find it a tad confusing given how the literature states things in other areas of the game.

strictly speaking the fact that some of the invocations specify warlock levels implies that the ones that do not specify warlock levels would have a wider scope, which could only mean character levels. In the tasha's rules it doesn't actually say to use warlock levels, just that you have to be a warlock which again implies scope

given that it is either delaying class progression two levels or burning a feat and one level, it seems like it would be a fairly reasonable tradeoff if a player just really wants to throw arcane eyes everywhere and become Argus

probably take hexblade for the seventh level on a barbarian and then use the feat to get relentless hex and start teleporting to enemies for extra damage

Quote from hkugtcdxjyktfx >>

strictly speaking the fact that some of the invocations specify warlock levels implies that the ones that do not specify warlock levels would have a wider scope, which could only mean character levels. In the tasha's rules it doesn't actually say to use warlock levels, just that you have to be a warlock which again implies scope

given that it is either delaying class progression two levels or burning a feat and one level, it seems like it would be a fairly reasonable tradeoff if a player just really wants to throw arcane eyes everywhere and become Argus

probably take hexblade for the seventh level on a barbarian and then use the feat to get relentless hex and start teleporting to enemies for extra damage

As mentioned in  the post just above yours, in the 2014 Player's Handbook the level requirement is explicitly for the level in the Warlock Class. 

The Warlock Invocations from Xanathar's Guide to Everything also explicitly say that the level requirement is the level in the Warlock Class.

The Warlock Invocations from Tasha's Cauldron of Everything are the ones that include the "Xth-Level Warlock" phrasing in their prerequisites, so that's also intentionally Warlock Level.

So at the time that the question was posed (more than a year and a half ago), rules as written, all core book published invocations explicitly cared solely about levels in the Warlock Class.

The 2024 PHB uses the "Level N+ Warlock," phrasing for its Invocation Prerequisites, so it still only cares about levels in the Warlock Class.