You can still use them if you want, but browsers, etc. stopped supporting them a while ago. They're just kinda irrelevant except for a tiny niche now.
Sure, maybe podcasts use it as an underlying protocol, but users' experiences depend more on the front-end client (some app) than the playlist protocol. It's not really a discovery protocol either; turns out having a central index like Spotify or the podcast apps do works better anyway.
I think it's irrelevant in the sense that it could be trivially replaced by app-specific implementations, and few people would even notice anymore.