I had this happen to me too, after I'd upgraded to 2.249.3 and tried to upgrade all the plugins. But in my case there were about 30 plugins showing as unavailable. I had to manually download and install a handful (AWS SDK -> Jackson -> SnakeYAML) so that my custom plugin had the version of the AWS SDK plugin that it needed.
I just went back to check the list again (two days later) and everything is showing on the Updates tab as usual, ready to install, nothing "unavailable". The only thing that's happened since I last looked was a restart (although I had restarted around five times while installing each plugin manually above, and it definitely didn't help).
Perhaps there was some periodic task that refreshed the metadata to fix the problem?
bryceman: just curious, has your list fixed itself as well or do you still have the problem?