Out of curiosity, what do you mean by genre?
To me, I'd say, it's a bit more like... the death of a platform. Not that I'm saying this platform is dead, just saying that I think that the genre is very much alive, especially with the increase of interest in TTRPGs.

Just a few scattered thoughts:

Personally, Discord RPing and Forum RPing feels very similar to me - the only difference is the ability to format posts in a more customized way using BBCode. I usually write around four to seven or more paragraphs, and this length does well in both Discord and Forums.

Writing on mobile is way more manageable on the Discord app than on the mobile version of a website.

One-liner RPers might like the speed of Discord more, as the responses feel very real-time. I can also see the appeal of OOC chat in Discord servers.

I suspect that some Discord RP communities can also feel "incestuous", if you're in more niche fandoms.

Reddit roleplaying is something I have never considered or tried, and it seems like a terrible platform to write a long, continuous story on. I am curious to know how popular this is, and why it's popular, at least based on your observations.

All that said, I'm actually here for the specific RPs I've found on this site. It feels like Discord RPing revolves more around fandom-type stuff, and in forums like here, there's a bit more opportunity for original settings and characters to flourish.