Resurrecting a dead torrent tracker and finding 3M peers

Is there a chance if someone bought suprnova’s domain (if available), which closed in 2004, someone could finish their download if a seed were still active? Does it matter if this was pre-DHT, or does DHT go historical to cover old stuff?

Yes DHT is "historical", in the sense that it doesn't care about when your torrent was created, just the infohash.

However, most torrents created for private trackers have the "private" flag enabled, which excludes them from DHT and PEX and a few other things. You can remove this flag yourself, but you're depending on a seeder doing the same for DHT to work.

DHT works as long as the client is configured to use it, so if that old seed upgraded their client they might end up automatically sharing the metadata over DHT.

Yes, this should work, in theory.