Recently wefwef.app added an option to add the link to all of your reddit subscriptions (multireddit link from old.reddit) and search for replacement subs. You can pick which communities/instances you want to subscribe for each sub.
You should know this because it can make your life easier if you want to search for communities you’re familiar with.
Tried it out, mostly found some dead communities with a few subscribers at best
The subscription number refers to how many people from your instance have subscribed, so if you’re on a small instance, or it’s a niche topic it can be misleading. You’re right though, there are definitely plenty of dead/empty communities around.