As far as I can tell, Relays[1] are the glue that makes ATProto work performantly. I think they're supposed to be content-agnostic — they just shuttle data through, reducing the number of services each AppView needs to be aware of.
As the blog mentions, the big improvement vs Mastodon is that Relays, AppViews and PDSes are separate services with their own distinct scaling demands. It's a rather beautiful solution to a system design problem.
Yeah, Relays are one way to do that. I've mostly skipped them because they're an invisible optimization and there are other strategies. E.g. many smaller apps today rely on Constellation (https://constellation.microcosm.blue/) instead of building their own database index, so they don't use a Relay at all.
They do remove content directly from relays. They claim they only remove content that is illegal to host, but I don't know how true that is, and there is always the risk it could change in the future.
https://docs.bsky.app/blog/blueskys-moderation-architecture#...
I want the bsky org to be able to choose what content they host (and I think the internet would be a better place without section 230 protections allowing hosts to ignore the content they distribute); the promise as I understood it was that relays could be hot pluggable. If someone stopped carrying content (maybe it was illegal in /their/ region and not yours) you could failover to another relay.
However there is very little incentive to mirror any of the firehouse if someone else is doing it for free.
If that becomes a massive problem - host a relay with different moderation policy.
The scale-down floor for running an ATProto relay or appview is much, much higher than the floor for running a fediverse instance.