This is a feature request to add a track changes feature to kate similar to popular word processing applications such as LibreOffice Writer. This feature is important for collaborative editing of files, especially prosaic work like technical or scientific papers.
Actually I think that is out of scope. We have no "own" file format, there is no way to annotate this to a text file (without adding some extra file or other hacks). Actually, that is really the thing you should use some version control system of you choice for, e.g. git/svn/cvs/....