When someone does a retag that removes one tag and adds another, almost certainly they were replacing a bad tag with a synonym that's in wider use. If a tag synonym proposal already exists for that mapping it's even more likely. It might be useful to either
- Automatically cast an upvote on the proposal for that user
- Notify the user that the proposal exists so they can cast an upvote if they want
I'd like it to be automatic, ideally, but I think most people won't, so just alerting the user that the synonym proposal exists would be sufficient. It might even be worth prompting the user to create a synonym proposal if there isn't already one, but that could get annoying if there's a lot of false positives; I don't know how good of an indicator "the retag removed one tag that had few uses and added a tag that had many" is for "the former should be a synonym of the latter"