As the main way to interact with people online? Yes, they are. There are more convenient easy, and centralized alternatives.
As a way to discuss a subject in depth? No, they are as relevant as ever.
Chat services are good for quick interaction, but they are lacking in anything long term. Not to mention most of the time Discord channels just end up in endless meme spam and just a handful of people are talking. I prefer to have them as a tool when I have to collaborate on something and to have them as a "phone number", if somebody wants to contact me they can use those.
The social media sites /link aggregation sites like Reddit and Twitter are good if you are looking for an endless feed of things, but they are lacking when it comes to search functions and archives. You need to be active when it comes to seeking out content. Reddit is just a big popularity contest, so the community is weird in certain subs. Twitter is really tribalistic at the moment also Twitter blows for art if you are a consumer, because unlike on dA or Tumblr and other dedicated art sites it will be hard to find the pictures if you are not there when they are being posted in the hashtag or you are not browsing the twitter feed of the artist...
I like the fact that on forums you can have longer posts and what you are making are there, and that is good if you want to see how a story was developing or just want to read the "effort posts" of the past. It is not really good for chatting, it was never the intended functionality. That is why Private Messages were invented back in the day.
Also let's not forget the cousin of the internet forum, the image board. They are alive and well. They are not identity based and permanent like old school forums, but the community around them made archiving possible and their unique way of doing things are keeping them relevant. The only thing they suffered from the rise of Discord was the weakening of "general threads" which are recurring threads for one specific sub community. The MLP fandom itself started there on the random and cartoons and comic sections after all. And pretty much a ton of internet culture as well.