I have been running a Mastodon instance since like 2016/17 and this has been quietly happening for the entire time I've been on the fediverse. (I can't check the exact date right now as I'm in the middle of upgrading it.)

Do you want to be in the Anime Girl Who Posts Nazi Memes Fediverse? How about the Queer Furry Fediverse? Or maybe you'd rather be in the Mocking Shitposts Fediverse? Perhaps you want the Everyone Has A Photo Of A Human And Thinks Federating With Facebook's Activitypub Is Actually A Good Idea Fediverse? Or how about the TERF/Gender-Critical Fediverse? Or the "Standalone" Social Site That Is Actually A Fediverse Instance With Federation Disabled And The Credits Removed In Violation Of The Source License?

Some of these Fediverses will happily talk with others. Some of them will rapidly defederate from others as soon as they encounter a place that clearly belongs to a Fediverse they are incompatible with. Some of them quickly get defederated from the Fediverses they are incompatible with. Some of them look at the #fediblock tag, some to keep aware of places worth pre-emptively blocking to make a chill place to talk, some to look for fellow people who have been cast out of someone else's chill zones.