I realize that this question has been asked on and off over the last 10 years that MB has been around, but so far I don't see any way one can un-do changes to tags when the metadata is saved to the files. Perhaps if it is saved to MB database only, one can un-do, but I don't see a way to do it.
I had a 'scare' earlier today when I wanted to make sure that none of my files had the 'enable custom sorting' box (in the sorting tab) ticked. I selected my entire library, went into edit, and sorting. I found the box un-ticked, so I hit the Save button, thinking that if any of the files had custom sorting ticked, this would remove it.
After I saved, nothing bad happened, but I suddenly had the fear that perhaps some of the other checkboxes on the sort tab had also been ticked. What would be the result of doing that? I suspect it would seriously mess up the database. But luckily, MusicBee leaves all the boxes un-ticked by default - unlike another program (Tag & Rename) I sometimes use for tagging, which leaves boxes ticked from the last operation performed.
So, at least MB is smart enough to clear the boxes so something bad doesn't happen, but it would be really nice to have an un-do feature; although I suspect that if writing tags to the files, this would be somewhat difficult. A cache of all changed files would need to be stored somewhere.
Of course the best way to prevent disasters that can cost many hours of work, one should always keep a good backup.
I do a backup regularly every night when I have been working on MB, but I think it might be a good idea to do a backup more often, when processing large numbers of files during a day. Maybe once an hour.