Navidrome can do this with smart playlists. I wouldn't call it recommendations - it's not smart enough from that - but you can absolutely create a smart playlist that will randomly pick a given number of songs from certain genres, that are (not) recently played, etc. that will refresh every time it's accessed
Yeah, the .view suffix always seemed strange to me. Not sure why it's even there at all. I didn't consciously decide not to use it either; I just am using a fork of an unmaintained Subsonic API client library and it didn't use .view.
Even weirder is if you look at the official documentation it looks like the endpoints don't have a .view suffix at all, and it's only in some example requests that you even see any reference to a .view suffix. E.g. looking at http://www.subsonic.org/pages/api.jsp#getPlaylists you'd have no idea that /rest/getPlaylists.view was even an endpoint.
The Subsonic API has a similar "jukebox mode" feature which some servers support, that plays music on the server's audio device instead of streaming it to the client app. So you run your server on a device connected to your audio system, and if you have people over anyone can queue up tracks. Gonic supports it, and Navidrome will once the next release comes out (or if you build it from source). I'm not sure which Subsonic clients have a UI for the jukebox mode though.
For something completely different, mpd was basically designed for this. But it's a completely different API and completely different set of clients, and streaming to a client is not really something mpd does. Of course you can always run an mpd server and Subsonic-compatible server side by side pointed at the same library.
You could implement the schedule play music with just a cron job triggering playback via the API