@benwerd I think the email protocols (SMTP/IMAP) fit the actively-used, open, and subscribe to published content criteria. I'm not even joking. 😊 Email newsletters are still a thing, and you can subscribe to content updates via email in feed readers like Feedbin. JSON Feed is worth mentioning as well. I don't know how actively used it is, though.
Sven Dahlstrand
I am an aspiring world's best uncle, co-conspirator with Sanna, and curious coder based in Jönköping, Sweden. If you spot someone hiking deep into the forest of Tiveden, that's probably me. 🌲