Under "Application Addons" you will find Plasmate, which populates all its items from https://download.kde.org/khotnewstuff/plasmoids/plasmoids.xml, as I understand. But, if you see this file, all items have relese date in 2013. Does it mean, the list wasn't updated many years?
What is this category for, if we have extra "Plasma Addons"?
This is a file that is stale on your file system. A quick action you can take to solve the problem is to just remove it from /etc/xdg.
The problem is that when a project decides to stop existing (or stop doing kns for that matter) they'll stop installing the file but most distros have the policy of not removing files in /etc/*.
*** Bug 395717 has been marked as a duplicate of this bug. ***
*** Bug 395715 has been marked as a duplicate of this bug. ***
Looks like this may be simply a matter of using QStandardPaths::GenericDataLocation instead of QStandardPaths::ConfigLocation in a few places.
Support for this got in with https://phabricator.kde.org/D19338.
Discover patch: https://phabricator.kde.org/D19340
Other apps and providers will be patched too once they can rely on Frameworks 5.57.