On Reddit, Reddit would randomly pick 50/100 communities from which it fills your home feed. Your other subscriptions do not matter until Reddit decides to refresh the list of subs its pulling from.

In theory, if you're subscribed to a bunch of inactive subreddits, your home feed is potentially being held back by these (since, as opposed to Reddit grabbing posts from 50 active communities, it's only grabbing posts from - say - 30 active communities with the remaining 20 being inactive and taking up the spot of your other active subscriptions).

On Lemmy, is there any downside to retaining subscriptions to inactive communities?