Building all of KF with EXCLUDE_DEPRECATED_BEFORE_AND_AT=CURRENT set, so dropping any deprecated API from the build, serves as a small sanity check to see if the future (like KF6) is well prepared and there is no hidden undeprecated functional dependency on deprecated API. Only backward-compatibel functionality should still rely on any deprecated API. Or otherwise the deprecation should be undone possibly.

No oversight over Plasma these days, so cannot tell what parts are still depending on metadata fetched from desktop files via KService. But given such API has been deprecated since KF 5.0 it might be time to complete any missing adaption.
So any changes done here are solely based on "it builds without deprecated API used" :)