While it’s never really caught on with any of the larger publishers, CriticMarkup is a great tool when working on edits between collaborators using Markdown or other plain text formats. It lets you specify insertions, deletions, substitutions, and comments, all with the option to include attributions, timestamps, and other notes.
This is a companion discussion topic for the original entry at https://brettterpstra.com/2016/06/08/multimarkdown-composer-and-criticmarkup