Plasma::PluginLoader contains methods for listing/loading of various Plasma related plugins (Applets, DataEngines, ContainmentActions etc).
A lot of this is based on KPluginInfo/KPluginTrader, this should be ported to KPluginMetaData/KPluginLoader.
It defines virtual methods that allow writing custom loaders. This seems to be mostly unused (only ShellPluginLoader (deprecated) and SimpleLoader (for tests)) extend it in KDE code. Might be a good idea to get rid of that then.
With https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/231 and https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/226 all public methods using KPluginInfo are deprecated.
What remains is the question whether we need/want to have all the virtual stuff in the future
https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/303 Took care of the virtual load methods.
https://invent.kde.org/frameworks/kpackage/-/merge_requests/25 took care of it's friend in kpackage.
The virtual standardInternal* methods remain, but I guess this can be deprecated too. I am not aware of any applications that use - or might use these methods. If anything they should use the KCoreAddons KPluginLoader/KPluginMetaData.
https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/369
Some stuff regarding the parent app filtering remains, but that is handled in T14730
Since there is nothing on the Plasma side to do and it only waits for KF6 branching, I have adjusted the tags accordingly