I think the conflict that's emerging right now post-BUR is the question of "under what circumstance was the Elite II tag allowed to exist?".
And based on what I recall from past discussions here on the forum, and elsewhere, the tag was clearly allowed to exist specifically because of the precedent set by Kantai Collection. Elite II (and the brief period Elite I existed) were treated in the same way as KanColle's Remodels, Azur Lane's Retrofits, Warship Girls' Remodels, GFL's MOD3s, Touken Ranbu's Kiwame, etc. - distinct achievable forms that strengthen the given character and in turn pair said form with new art, with those forms being added overtime - and anyone who actually knows Arknights would be already aware how wrong of a comparison that is. 4* and above operators in Arknights have the ability to access Elite II by default.
I know you're complaining on how it is hard to actually find the official art of Elite II units now, but fundamentally, thinking of usual Danbooru tagging procedures, had the full context of Elite II been known to all, I don't think it would've been allowed to exist under Remodel rules in the first place. Part of the reason why the Remodel-style general tags existed was because there was a guarantee that the person in that art would look different from their default (for the most part - see forum #203985 for an example), but that ultimately led to the question of "how do you distinguish between someone who is remodeled in a post if they are with someone that isn't", which ultimately led to the breakdown of those tags, especially to make it easier to find characters' default designs, as they've since been overshadowed by their remodeled forms. Meanwhile, as has been mentioned so far, there's nothing that guarantees a character will look different with their Elite II, and the approach that's increasingly taking hold on site is "if the character has multiple distinct versions in their source material, then each version should get its own tag", that leaves Elite II in a bit of a pickle.
Ignoring all the flourishes the official art may have, Elite II art can be divided in three categories: a) completely new look; b) variation on current look; c) completely same look. Those under a) would have to get their own tags, and c) would not, but b) becomes an iffy point. Mudrock is the perfect example of this - fundamentally, her Elite II art is just partially removing her suit, revealing what she actually looks like. If you were to ask any other Danbooru tagger on site, they'd be telling you that making a tag for that is preposterous, because it's just her without her suit on. The only ones who wouldn't react like that would be the Fate/Grand Order taggers, and that's only because they tag the default appearance distinctly too.
In fact, when it comes to valid comparisons, F/GO is way closer to Arknights - First/Second/Third Ascension are the closest parallel to Elite 0/Elite 1/Elite 2 on site, being a core feature available to all servants, with each ascension giving art and designs. But because they were a core universal feature (and probably other reasons too, insert complaints about Fatetagging here), ascensions never had general tags. By the time the idea was actually raised, see topic #17300, remodel-style general tags were already a thing, and so an initial pitch was made to use that format... until the current admin stepped forth and basically began the domino effect that would eventually kill remodel-style general tags: ''If a character has multiple distinct versions, then each version should get its own tag.'' So, with that, further bolstered by topic #20723 and topic #21023, ascensions received their own tags. But the big, big difference between F/GO and Arknights is that, for the former, ascensions can still be discerned more often than not - meanwhile, more often than not, Arknights' Elite IIs are not discernible from their defaults, so if you were to pitch the F/GO approach, most cases of (elite_nulla) and (elite_ii) would be (near-)identical. This isn't even to mention that not all ascensions are tagged to begin with, because the emphasis is on the visual differences between ascensions (something tangibly taggable), not just tagging them because they are ascensions.
So, yeah, by all accounts, the tag shouldn't have existed to begin with. If you wanted a way to search up Elite II official art, then a pool ala what blindVigil suggested seems like the most valid option, and if actually discussed years ago, would likely have been the way to go. Elite II parody art, meanwhile as suggested, could be covered with a tag because it's a recognizable enough style that's distinct from general Arknights style parodies.
