I’m working on an additive trust level plugin that adds/deletes from a group depending on membership on a set of other groups (e.g., membership in paid and special also gets you into the paid-special group).
It’s very strange that TL promotions will trigger an :after_save event, but TL demotions do not trigger a :after_destroy event, but I’m working around that with DiscourseEvent.on(:user_promoted)
But that doesn’t work because the trigger gets called before the trust level is updated:
I submitted a PR: https://github.com/pfaffman/discourse/pull/205