There isn't a single .desktop file that I can see, that sets AllowDefault=false.
So we might as well remove the whole concept (of an app associated with a mimetype, but the user *has* to use RMB / Open With FooBar instead of being able to left-click on a file), and simplify quite some code.

I looked at whether the git log had a use case for it, and it was there already in 1998 when Torben created kio based on some KDE1 kfm code, which I don't think was ever imported into svn/git ;)

These days it would be incompatible with XDG mimeapps anyway.