While I might be an edge case, I just ran up against this.
I noticed extra "+ Reply as new Topic"s appear while I was searching for how this (advertised) feature worked and suddenly had topics open both with and without them.
In further searching I did find Understanding Discourse Trust Levels but some indication that there was a change to what I could do would have helped me, particularly because it’s introducing a nuance of Discourse compared to the old forum model. PM seems ideal.