From what I see, y'all have created a real mess here.
One thing I don't see a lot of mention of is that modern comments also deletes the option to show all revisions inline. That is really annoying, because then the real estate for the comments/balloons constantly sits on the right side of the document, even minimized to 0.8". I read scripts, for instance, and every inch of screen real estate is valuable and needed.
I was on 16.54.21092901 and updated today to 16.54.21101001
What I've found, starting with opening Word after enabling modern comments was checked in a previous session
-- If you uncheck allow modern comments in the new session, but you leave Word open, the show all revisions inline option does not return.
-- However, if you close and then reopen Word, then show all revisions inline is available.
-- If you then enable modern comments, the inline only option does not return till you close and reopen Word.
I assume it won't write this particular preference to normal.com (or somewhere else) until program close (though that also seems ill-advised for something so integral to many workflows.
The current situation is clunky, but it works, I guess. But Microsoft has said it plans to do away with the ability to not enable modern comments. At that point, I can only assume the show all revisions inline option also goes away. That would be very bad.