Is there any documentation on Subeta's sAPI?

Nothing available to users, as far as I'm aware.

You might want to browse through the Site Updates forum for more technical info. But that stuff is still pretty general, aimed at people with little computer knowledge.

there's also the server status and change log pages but those haven't been updated since last year so i don't think they're in use anymore.

So sAPI isn't public? That's sad, because it could really help the Subeta fanpages.

I know there used to be a public API, but I'm not sure if it's still maintained.