There is some documentation here, although I'm not sure how up to date it is.
Currently there will probably be more pressure to drop old APIs with HAF approach which kind of allows everyone to build their own customized API, however in case of front-ends it is actually beneficial to have something common like Hivemind (less repeated work and also ability for users to switch to different server in case the one they are using has difficulties). Current Hivemind APIs have many problems though, especially with efficiency, so it might finally be the time to design something new from scratch.
Thank you for explaining @andablackwindow.