A week or so ago, I went off to your stable AppImage page:

https://binary-factory.kde.org/view/App ... ableBuild/

and downloaded whatever was the recent version then. When I ran it, the Import options were all grayed out, even though I'd configured the Plugins page in Settings. So, I deleted that version and downloaded the 9 month old version from my Fedora repository. Perhaps I did something wrong, so I'll ask: do I need to download plugins for the AppImage versions from somewhere? If so, where do I get them?

I needed to check if a new version fixed an issue I was having. So, I downloaded the latest stable version. All the plugins I needed were present and worked. Nothing more needed downloading.

Did you uninstall the Fedora package before you tried the AppImage again? It could be, that the AppImage is missing some files that get installed with the distro package. And no, you should not install additional plugins for the AppImage. They are contained (but might be disfunctional due to missing files, which we need to find out).