I can’t find the reply as linked topic when I’ve looked. And even when people are telling me, I don’t think it’s visible on mobile.
When I started using Discourse, it was a key feature that I used. I created a topic that was an assignment and they’d reply as linked topic to give their answer (and also add a tag, if memory serves).
Also, reply as linked topic seems like a solution to not having threads. I have a site that has topics Wyeth thousands of replies; maybe if linked topics were more obvious they could split off into more topics.