@malte @evan I think @rwg is working on that history. Some of it is touched on here:
https://fossacademic.tech/2023/10/15/APnonStandard.html
https://datatracker.ietf.org/meeting/120/materials/slides-120-rasprg-robert-w-gehl-activitypub-a-non-standard-standard-presentation-slides-00.pdf
... but there is more to be said. And yeah, maybe I should do my own writeup, some time.