There are, like, several problems here and I don't even know how to split them up properly:
* Several styles previously found in 'interface' or 'elements' or 'content' or something (I'm not really sure which, as I've never really used any of these to begin with) are now in legacy. This includes the toc-style catlist box and thumbnail icons that now override any skin-specific icons (T280292).
* 'legacy' loads by default and needs to be explicitly disabled, so even if one wasn't using it previously it still suddenly shows up and applies all this?
* Some styles previously in legacy apparently... just aren't there anymore? I assume this is where .plainlinks support was previously coming from? (T279693)
Related: While the toc styles themselves are not part of 'legacy' (I don't even know where they are, but the ToC remains messed up with monobook-esque styles (T280285) even when disabling the 'legacy' feature) ...which would be good, except where is it, and why is it also now default on?
This affects most skins, and I'm have some difficulty seeing how this wouldn't break any given skin that isn't specifically, intentionally working around it...