The old KCM enabled and disabled the kded (both autoloading and load/unloadModule) depending on global menu setting.
Since we never had global menu in 5.x before, it would disable the menu when you hit Apply.
This will break global menu for users without an obvious reason why.

Unloaded global menu module, hit apply, module was loaded.
Disabled global menu autoloading, hit apply, module autoloading was enabled and module was loaded.

This obviously overwrites when users disable this service but then, the service could/should always be running. It won't register a com canonical appmenu registrar service if it's not enabled, so apps will work just fine with kappmenu service running.