I find this one of the most bizarre desires of some YouTubers. There is no mechanism for discovering whether a particular subscriber is active. You can't go by comments or likes on their own account as many people are just lurkers but still watch videos. You can't go by metrics because YouTube doesn't share that particular subset of information. So you can either just wipe them out en masse, or start over. And in both cases, you're creating more work for yourself to avoid a problem that doesn't exist. Having inactive subs doesn't hurt you in any fashion. Sure they don't help, but they don't hurt either.
Spend your time doing things that are productive for your channel. Not wasting time on things that don't matter.