Oh, what also just comes to mind is the #Polyproto protocol. I think it may have been created in reaction to ActivityPub's complexity and issues around identity, not sure.
And another protocol in the works, inspired by AP is #Versia.
What I notice is an uptick in interest in #Hypermedia and using the open web standards as intended and best supported in browsers.
You see that in web frameworks that go for radical simplicity, like #Datastar at https://data-star.dev where the front-end offers intermediate-mode #UI and logic shifts to the back-end.
What I found interesting (for its concepts) many years ago when #LinkedData was still hottish, was the #Hydra hypermedia project.