Once upon a time ActivityPub came onto the scene of distributed social networking. Back then discoverability was (and I guess to some extent still is) a major thing to care about. It appears that almost all implementations, notably Mastodon and Pleroma, of the server side have a page for The Federated Timeline.
What this means is that if user @firstname.lastname@example.org follows user @email@example.com, all of the latter's posts will show up on the federated timeline of y.z, since that instance receives them. This means that users at y.z will be made aware of @firstname.lastname@example.org, and in extension of the instance b.c of course.
But then @email@example.com gets offended by something that @firstname.lastname@example.org posts! The post is reported to the local moderators (at y.z) and they have a look at it, and maybe check out the code of conduct of b.c and see that it is entirely incompatible with their own code of conduct. So they decide to block b.c entirely. No users on y.z will ever be notified of new posts from b.c and vice versa. To the great annoyance of @email@example.com and their potential followers on b.c, but whatever.
This is how instance blocking spreads and become a thing. It's not enough that some users on y.z block @firstname.lastname@example.org, because there are a number of other users on b.c that cause them distress too. It makes sense for the moderators of y.z to block the entire instance.
The federated timeline, which was once a way to glimpse into the fediverse and marvel at the many things and people there, has become a source of frustration and drama. What say we just get rid of it? You can discover new people via boosts or by seeing them take part in conversations that you or your friends take part in. A slow, organic, discovery process.
On big instances even the local timeline can be problematic for the same reasons. In a way the federated nature of ActivityPub is comparable to email; you post a message to all of your followers no matter which server hosts them. Except your own instance also publishes your post in a handy timeline (or maybe timesink) for everyone else to doom scroll through.
On smaller instances with a tight community the local timeline can be a precious, awesome thing. That doesn't mean it has to exist everywhere.
And the federated timeline should go the way of the dodo.
-- CC0 Björn Wärmedal