SUMMARY
After upgrading do KDE Plasma 6, older KDE 5 global themes, cursors, icons, splash screens, etc stop showing because they are not supported. This is fine, but we also lose the ability to uninstall these unsupported add-ons. Especially with Global Themes, these can get quite big in terms of disk space used. I myself had maybe a hundred of these themes installed. After upgrading to plasma 6, all of them are gone (which is expected) but I cannot uninstall them anywhere. If I open the "Get New" dialogs, and filter by "Show only installed items" it shows "No items found" even if I have dozens of them installed (I do, I checked their respective folders). The only way to remove unsuported widgets is to manually go into their respective folders and delete them, something that is very confusing for casual users especially considering the fact that we can in fact delete very important default themes/add-ons from these folders and break functionality. Users should not need to go delete add-ons folders manually. We also have the problem that these add-ons "get lost" after becoming unsupported, in a way that while they are still there using (what can be a lot of) disk space, the user has no way to know they are still there.

The "Add Widget" dialog from the desktop widgets works in a different way, and can be used as a great idea to solve this: While the "Get New" dialog for these widgets still cannot show unsuported but installed add-ons, the "Add Widgets" dialog does: it shows all the currently installed widgets, even the ones that are unsupported. These unsupported ones are greyed out and while hovering them with a mouse they show an unsupported message tooltip with a great user-friendly description. They have a "trash" icon on them that can be used to uninstall them.  This way the user knows that they are unsupported, but still installed and wasting disk space, and can safely act on this without needing to go browse folders and delete stuff accidentally.

My suggestion: Copy the "Add Widgets" dialog functionality by showing all of the installed add-ons everywhere (global themes, window decorations, icons etc) but have them greyed out and disabled. This way they can be uninstalled safely, just like any other currently supported add-on. They should also appear, also greyed out with an unsupported message, in the "Get New" window when filtered by "show installed items only".

STEPS TO REPRODUCE
1. install any plasma 5 (not supported on 6) global theme (or any other plasma 5 exclusive add-on)
2. upgrade to plasma 6
3. all installed addons are gone, user has absolutely no idea where they are, but they are still wasting disk space, just completely hidden from the user

OBSERVED RESULT
Everything is still installed but hidden without any notice or way to safely uninstall them

EXPECTED RESULT
Every installed add-on should still be shown, just greyed out / disabled and with an unsupported label. User should be able to uninstall these addons easily without needing to go mess with folders and files manually.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: from 5.27.10 to 6.0.3