ie. topic boards with ephemeral linear message threads

This describes a general forum format, but you might mean chan imageboards specifically. There have been federated imageboards for a while, but they're very niche and experimental and I don't see the value. The two examples I've heard of are NNTPchan (2015-present, NNTP protocol) and Fchannel (ActivityPub protocol).

There's the related imageboard webring, but there's no actual federated interaction between the boards, it's effectively just cross-advertising to allow easier discovery.