Thanks a lot for the bad rating
As explained there:
You can enable compatibility mode for outdated servers, the wizard should tell you that. (And the doc clearly state that if you server is maintained but does not support that, you need to contact me so I convince them), not put a bad rating based on false assumptions.
There’s a ton of work since a year to improve Subsonic API, and we now are building a public organisation to achieve that goal.
As you can see after a lot of works most recent updated servers do support the API.
So to resume: Subsonic API is fully supported including for outdated and no more maintained servers via the compatibility mode.