While composer (and possibly performer) tags are really obscure in most genres of music, they are pretty essential to keep a classical music collection organized IMHO. Support for these is lackluster, but as far as i could find the opensubsonic API spec includes it (see links below)

https://opensubsonic.netlify.app/docs/responses/child/
https://opensubsonic.netlify.app/docs/responses/artistid3/

I've been trying to get around this using the Album Artist and Artist tags, but it's getting pretty confusing and doesn't work all that well :(. I'm also not sure how many server support that tag, probably not yet a lot, but it has to start somewhere (i also submitted a similar feature request to my favourite airsonic server gonic)