From another meta [question][1] here, I guess this is a little-known feature.  

Look at a question page on MO.  On the right, below `BLOG, HOT META POSTS, Related` etc., you will find:

[![image][2]][2]

Click on that to add yourself to the "feed" for that question.  How it works, I think, depends on how your computer has been configured.  In my case, these notifications arrive in my "News".

I note that there is a "user feed" at the bottom of a user's profile page, so it seems you can sign up to be notified of whatever that user posts.  Look around.  You may find other feeds as well.

  [1]: https://meta.mathoverflow.net/q/4089/454
  [2]: https://i.sstatic.net/Dg1qd.jpg