Nearly all of the search options sort your front page by activity, so removing old communities won’t make active ones stand out any more. If you’re searching for new communities and fail to notice the last message was 3 years before you posted your 900-page essay, that’s on you. Even if you make the background of stagnant communities bright red, there’s still going to be someone who complains that they somehow “didn’t get any warning”.
Retaining old content has value, you wouldn’t believe how many answers I’ve found on 10+ year old reddit posts that have long since been archived. Information is valuable, it should never be removed unless someone is being harmed by it.