When I updated MyBB to 1.8.27, the RSS Syndication has now stopped picking up any of the latest posts despite having daily postings. This is for all three forms of RSS (RS 2.0, Atom, and JSON). In fact, the syndication feed considers the last post all the way back in July.
What's your URL? The most recent major changes to this functionality would have been in 1.8.21 over 2 years ago, so shouldn't be any issues in 1.8.27 causing this.
I sent the url, but I did make a test forum that is not private, and the RSS Syndication DOES work for it. So I wonder if it's a permissions issue, though that never was a problem before the upgrade. I picked "Syndicate All Forums."
Another update; the RSS Syndication is only giving the updates of the current threads. It is NOT giving any recent post feeds like before. For example, the entire feed only shows all the threads created recently, not posts. So it is working, but the nature of the RSS has changed.
It will respect permissions, so if the forum is private it won't pull anything through. It also only pulls through threads, not posts - it's never done that so I'm not sure what you've had set up previously. Did you have a plugin to extend this or edited the core file?
So I did more digging and apparently, last year I made a change to change the RSS to show posts natively, not just threads. I used the solution below. The upgrade had me replace the Syndication file, which explains why the RSS functionality went away. I just reimplemented it and it's working again. So I consider this resolved. I merely forgot I had made that change.
https://community.mybb.com/thread-52493.html