retag button is gone and only
edit remains. This has made a few negative changes for users with the retag privilege but not the edit privilege.
It is now necessary to enter an edit summary even after changing only tags.
I don't think this is necessary. From my experience in the suggested edit review queues, most users will use a message such as “added a tag” or “edited tags”, none of which provide any useful information. While it is good to be able to enter an edit summary, it should not be required.
In any case, the recorded edit summary in the question history is always “edited tags”, no matter what the user enters. (Noted by luiges90, and I confirm.) So this is definitely a UI bug.