Surely as Administrator (Steven) or Moderators (psychoadept and phred) you can edit a post? If so, could you not put a bold message at the top of the first post of such topics to say that this is now non-operational and not being supported?
If this is not possible, how about creating a new section within MusicBee & Add-Ons called Defunct Plugins (Unsupported) or some such and move affected posts/threads there just like the Mods do with misplaced posts in other threads? Criteria could be no log-in from originator for 12 months+ or repeated complaints of broken links? If the originator is still out there and takes exception, they'd pretty soon get in touch! Same could be applied to any of the subject sections.
Just a a thought?