When I first read through the API I thought I was missing something because I couldn't find what I assumed to be an obvious endpoint for a music streaming platform. I think it would be smart to assume this will be an infinitely de-prioritized issue and the Web API will remain something cool for small "toy projects".
While I think it sucks that no progress has been made on this, I think the larger problem is the veiled curtain that exists between internal development and the public. I'm assuming it's a cultural problem with the company though that any individual developer probably couldn't solve. In an ideal world we would hear answers like "A developer on our team tried to get this to working last week but it ended up requiring massive architectural changes that wouldn't be feasible right now. We are planning to release a new version of our API in a few years that should support this". Or maybe "Unfortunately, we believe this is a feature that we would like to monetize for corporate partners. If you would like to partner with us for this feature please contact our sales team."
Anyways, I think I'm crazy for imagining they would ever say something like that even in an alternate universe xD.
As everybody said, we're stuck with unadapted technologies.
Polling is not eco-friendly, nor efficient and subject to rate-limiting (that's fair, though).
But please provide us a way to access an already-existing and stable API. Discord has a dev-friendly system and you can interact with the app really easily thanks to their APIs. Now it's your turn, become dev friendly and the de-facto standard for building apps revolving around music.