I would expect every relay to have everything (modulo different opinions about illegal content). Why do you think they would have different views of the network?
Well, they'll try. The relay plays a similar role to a search engine, though without the ranking. Why do different search engines have different content?
As others have said, spam and illegal content are the most likely reasons.
We might also compare to email delivery; maybe ideally every message would be delivered, but in the real world, they aren't, and open relays are a hazard.
It's virtually guaranteed that different relays will take different views regarding disinformation, harm, etc.
This is probably good, too. I personally don't want my hand held, but recognize that others justifiably want someone else to filter out gore, or even opinions they find sufficiently off-putting.
I don't want to ram particular content down anyone else's throat, but I'm 100% opposed to anyone making that decision for me.
Is it possible for a relay to filter individual posts, or do the Merkle trees prevent that? (They would if they need to be signed by the originating user.)
I'm part of a marginalised group that is often considered to be illegal in many countries, or at the very least, very off-putting. (In 50 years, perhaps, things will change and we will be more accepted. To be specific, it's a grey area in the U.S.)
I'm interested in BlueSky but the relays are a worrying "point of failure" if they can just block me from there. I'll be more interested if there are multiple relays that I can use in parallel, with at least one being friendly to us, so I can still participate. Currently on the Fediverse it mostly works since we can connect directly to other servers, and there's no SPOF. But I would love to use BlueSky due to the better protocol design! Just more relays would be good.
... Wait, can we self host relays? And is it plausible for a community to do that? I couldn't find anything about it.
You can host your own relay but if you want accurate likes/replies/etc you need to crawl the entire Bluesky which could be quite expensive in the future.
