That's definitely worth keeping an eye on, but actually its not that bad. Take this with a grain of salt, as I only have a user view, not mpv developer. I consider the JSON IPC API quite stable, well documented through mpv(1) and https://mpv.io/manual/stable/. Almost every command-line option documented in mpv(1) is mapped 1:1 to the JSON IPC, so mpv(1) documents both the command-line, and, the JSON IPC.