In preparation for Jenkins PR 5778 I'm proposing a bunch of PRs for a variety of plugins that still look for paths rather than icon classes, and a bunch more I forgot to add to my list linked (will do so later).

Skimming over matrix auth, I noticed that ValidationUtil#formatUserGroupValidationResponse does still lookups for the path of PNGs which are subject to be removed. A few jelly files are affected as well.
I don't know whether I have time to squash in a PR for this plugin as well, at least not this year anymore, I'll drop the task here, in case someone else wants to pick it up.