It's up to the individual server who they federate with.
If you want a gated community, or you want to cut ties with an instance that is full of spam or unwanted content, that's still possible.
In the swarm of starships, you aren't obligated to dock with everyone.
Yeah, it's read-only.
IMO that's still better than other sites, where they don't federate at all.
On Reddit, you can screencap something, and paste a static image. You can't see the comments underneath at all.
I expect instances that don't fully federate will grow slower, so it won't matter, most content will be on instances that are open to fully connecting with others.