This directory has an enormous amount of files with the following naming pattern: mate-applications.menu.undo-[0-9]+ Can I safely delete these files? Are they backup files?

I noticed this the other day too. They are no harm, as far as I know. It would be a good idea to keep a few as backup, since I ran into a nasty bug when editing the menu destroyed itself! :scream: Life Is Strange™ was the culprit, all because of that ™ (non-ASCII) character.