I think what you are referring to is not content so much as that new communities needed - in the past - to have been added by someone before others could find them in the community listing. This was not restricted to admins though & people like Blaze had an army of alts and did much of that, as did I on my own instances as well. This was necessary before the first post could come in from each new community, needing to be performed on a per-community basis.

However, PieFed made an improvement to not need to do this anymore, and then Lemmy followed suit as well, and now the whole process is much easier. Although it gives Lemmy.ml specifically veto power in that if a community is banned or the instance it is on is defederated from Lemmy.ml, then the automatic pulling in of the community name is no longer done. Technically admins - or again, I think anyone - can always add it the old way, but only if you know that it exists and how to make that happen (which isn't hard but requires crafting a URL rather than mere clicking on links). Also, the admins can designate a different instance than Lemmy.ml, though Lemmy.ml is set as the default, so this procedure is opt-out.

Those PyLova devs are hella impressive!!! Rimu seems less interested in the Threadiverse these days and more in interoperability with Mastodon, despite how most of us seem to not even want that, and Mbin already exists. It's okay for him to work on whatever he wants, but it makes it all the more impressive to see how responsive the PyLova is to the needs of the people on the Threadiverse.

I don't think that most instances do delete old content. Lemmy.world self-reportedly has content from decades in the past from nodeBB but I think their database got corrupted and I don't trust that. However, here is an example on my own instance https://discuss.online/post/5497584 showing the retaining of a post to a community on Lemmy.ml from 8 years ago.

The PieFed thing was once again another of Rimu's rants against "unserious" meme content. He is obviously entitled to not feel forced into providing unlimited hosting for images, but I am saying that the limitation built into the PieFed software and enacted on e.g. PieFed.social may not be universally applied. I could well see e.g. lemmynsfw.app choosing differently to retain every image forever. Unfortunately there's almost no way to know short of directly asking the admin team, who may not even be aware themselves what settings are in place on their own machines.

This is a limitation of all Fediverse software that I've seen, including Lemmy and Mbin, so not just PieFed. Though PyLova does have the most transparency of all of them... and yet I haven't seen even PyLova expose whatever these settings are to the public. Even though someone might make a choice as to which instance they want to make an account on (or even start up their own) if they could find the answer.

I think even subscribable blocklists would only be the first part of an overall answer. Like UBlock Origin, it's going to require people to manufacture and maintain those lists. But... at least the software will support that now, so yes indeed this is the very important, crucial first step!!