I can only think of Futaba-style discussion boards, that is, 4chan. On 4chan threads are linear, but you're supposed to reply to posts by mentioning their unique post nº. For example, the above comment would want to mention >>33566411 and >>33566389. You can mention as many posts as want. The posts get a backlink, so the thread is a navigable, true graph structure. Reading discussion on 4chan can be confusing as a deep chain of replies can branch out to mention a recent surface post, and to understand what's going on you have to both delve down focused reply chains but also be aware of incoming posts that are displayed cutting across any one discussion.
Here is a typical 4chan thread that demos this:
https://desuarchive.org/g/thread/87053939/#87081210
I chose to link a post where a picture visualizes a 4chan thread in a directed graph. Also, a screen above there's a "Thanks." to several other posts, in one post.