@hugh@ausglam.space A few do, Pleroma historically supported it. Not sure if they still do.
Part of the problem is that it's kind of unwieldy to work with. C2S apparently requires developers to handle all logic on the client side, rather than the server. I've also heard that there's vagueness in the spec on how to do certain things.
The Mastodon API ended up being a lot easier to work with, and overtook the client ecosystem. It actually ended up becoming so popular that most Fediverse clients and platforms have adopted it. It's also one of the factors as to why so many Fediverse platforms are full-stack servers, rather than clients.
I personally still think that C2S has potential, it just requires capable hands to implement.