Shockingly, this isn't even listed as a featured bullet on the plan list. https://developer.mozilla.org/en-US/plus#subscribe The only bulleted advantages of paying $10/month are, "Early access to new features" and "Pride and joy."
As others have noted here, none of the "Plus" features are very useful: Collections, Notifications, and Offline support. Collections are just bookmarks, which all browsers do for free. Notifications are pointless, because all of the pages are on Github; you can subscribe to notifications there (but why would you even want to??). And I approximately never need to use MDN when I'm offline.
We know how to do this "correctly." MDN Plus should be a VIP pass to access the MDN team, via a private forum and/or chat room. Talk to (survey) the paying users for what new material they're interested in, and provide that.
This is how basically all Patreons work. People buy those subscriptions like hotcakes, they have excellent margins, and the subscribers are reliably very satisfied with the result.
EDIT: Buyer beware, I just signed up for the plan, and all it does is add a "Feedback" menu item that links to https://github.com/mdn/MDN-feedback … but that's a public repo. Anyone can file an issue there. I certainly did, and I'm not happy about it. https://github.com/mdn/MDN-feedback/issues/43
There's no Discord, no forum, no mailing list, no scheduled upcoming fireside chat… just a public Github repo where you can file an issue and hope for a response.