Isn't this a spoiler, the same way how almost every single post where Ranni is wearing the ring that the Tarnished gave to her is marked with spoilers?
I kind of agree and kind of disagree with the implication.
When I'm looking for art of ranni_the_witch, I'm only interested in her "large" doll form, not the small one. They may be the same person, but it's two fairly different bodies (only size wise). I personally don't know any similar examples, so currently it's hard to judge.
The only example I can think of is ainz_ooal_gown and his "alter-ego" Momon, who doesn't doesn't have a tag on danbooru. It's basically a "good" version of Ainz, that is always in armor, yet all posts with Momon get tagged with ainz_ooal_gown.