Piefed is another instance type that is growing in popularity, and a few instances are now turning on the API. As such, apps are also implementing support.
It would be great if Boost would support it too. Hopefully the extra development overhead isn't too bad.
Some recent discussion: https://piefed.social/post/817564
The page for developers:
https://join.piefed.social/docs/developers/
The API for third-party apps (frontends, bots, etc) is 95% the same as the Lemmy API.
That same link: https://freamon.github.io/piefed-api/
Different codebase, as far as I understand it. Not sure how different the API is (well they claim it's 95% the same).