It is not advertised in the interface, but this Discourse instance exposes RSS feeds. I wanted to monitor the Links category, so I modified https://discourse.nixos.org/c/links/12 to https://discourse.nixos.org/c/links.rss and fed it to elfeed.
Other categories are probably similarly RSS-ified.
Tip: many RSS clients will correctly follow the <link /> exposed in the page’s HTML. So putting the “normal” page addresses in your RSS reader might work.
I don’t know how smart readers are expected to be, but Announcements page source contains:
<link rel="alternate" type="application/rss+xml" title="Latest posts" href="https://discourse.nixos.org/posts.rss" />
<link rel="alternate" type="application/rss+xml" title="Latest topics" href="https://discourse.nixos.org/latest.rss" />
<link rel="alternate nofollow" type="application/rss+xml" title="RSS feed of topics in the 'Announcements' category" href="https://discourse.nixos.org/c/announcements/8.rss" />
That is: the third link is correct, and the reader must check the title to filter the other.
So if you happen to get all posts instead of just a category’s posts, that could be the reason.
The client I’m using will suggest all of those. So you can pick from those titles. But yes, highly client-dependent. I suggested the tip not to you necessarily, but as a hint to “just try it and see” to other peeps 
awesome! added to my Nix feed thanks