As someone who uses Git for technical writing and Word's revision system for fiction that goes back and forth with an editor, I mean, sure, it's sort of a merge request, but you need to place a higher value on the "goes back and forth with an editor" part of the requirement than I think you are. :) An editor suggests changes, sometimes by editing directly and sometimes by leaving comments, that the author can accept, delete, or modify, right? If we're talking about technical writing that's already in a Git repo, then using a PR review system like GitHub's is an acceptable substitute. If we're talking about somebody sending a story to the New Yorker, we're not.