That’s not how real world works 
(And as the tags says it’s already implemented for Navidrome).
Each function have differences and sometimes multiple versions. Like to sync I need support for empty queries to search3 endpoint there’s 3 different ways depending on servers to send that, and handle the fallback for servers that do not handle correctly that case.
And that also means that I need to fully test and maintain a compatibility matrix for every single functions and all the servers. Something that I absolutely not have time for.
As you can see : Explicit server name and version it was already a lot of work to have servers expose what they are. And there’s still the majority that do not expose their version yet. Meaning I can’t even build that compatibility matrix properly 
So until the work is done with API extensions and servers proper handling of things, every variation is a support burden and time loss for every future changes to manage regressions.
you can have a feature set for each server
I support Emby / Jellyfin / Kodi / Plex / Android / Subsonic. That’s already 6 (8 actually as 3 modes for android) different set of features to maintain and evolve with their versions.
Adding 20 more for 20 different Subsonic servers is just not possible. Specially for edge cases.