It would be nice to see people engaging with old posts when they stumble across a community and subscribe to it.

One barrier that will make this difficult is that instances only get a community's feed from the moment they first subscribe to it, if that community's home instance is on another server. So if you're a user on - say - leminal.space and you're the first person on that server to subscribe to - say - Musicals@kbin.social then you will not see any of that community's old posts, only posts created (or boosted) after you've subscribed. This makes it difficult to engage with old content unless other people on your instance have been members of that community for much longer.

This is one of the issues with the fediverse model that doesn't exist in a centralised model like reddit. And, sadly smaller, niche communities are the ones most likely to be affected by this limitation, because they're the ones least likely to be federated to a large number of instances. It makes smaller, less active communities look even more inactive than they actually are.