@Kindnesswinsall:
To me, arguments in the comments about character-achetypes would seem to be potentially more caustic than arguments about plot-archetypes, but I admit I have no evidence for that, so eh.
However, I'm not sure I agree that Yandere tend to be main characters? In the manga I read, at least, they're really much more likely to be side characters—either for comedic relief, or to provide dramatic tension of some sort (or both).
IMO, the firmer argument for a yandere-tag would seem to actually be how
niche it is to actually have them play a starring role—most people don't prefer (semi-)stalkers starring in their romances; the other most common
-deres would seem to be less polarizing.
That leads to a different slippery-slope problem, however... We don't tags for
every plot element that's an "acquired taste", do we? There's quite a lot of them.