Discord is better than forums for casual chatting and sharing memes. As far as acting as a repo of frequently asked questions to be left open for information to be amended and appended, forums are far superior to Discord. It's surprising that there are people who think Discord is better in every way when, to me, it's clearly not true. I wonder how many of those Discord-type users actually grew up with the chat rooms and forums/BBSes of Web 1.0. For those who didn't, maybe Discord superficially seems like a huge advancement over forums.
The sad thing about forums is that forum software seems to have given up on itself. People clearly prefer the [old] Reddit experience over the experience provided in vBulletin, phpBB, Simple Machines Forum, Invision Power Board, Discourse, etc. These softwares remained in the past, and I think that's part of what's lead to their demise in terms of relevancy.
The most interesting class of communication platform to me is 2/4chan style text/imageboards. Depending on the speed of the board they can feel like near-real-time chat (with copious memes and verbal banter), or a forum (complete with megathreads for beginners on a topic), or something exactly in-between
For information retrieval it's quite terrible. Generally even worse than Discord due to the archives being on random overloaded sites. OTOH it leads to a culture of survival of the fittest ideas ("meme" culture, I suppose), where instead of upvoting a post or image you like, you just repost it. Like a big shared human cache, or like human memory I suppose
The general lack of persistent identity has a profound influence on the culture of course. People tend to focus on the negatives that it brings, but it can also be refreshing to have pure and frank discussions with no ego or profile snooping or cliques. And just generally the feeling of low-stakes casual chatting, rather than remembering at the back of your head that everything will be on the record against your username
So I grew up with chat rooms/forums/bbses... and am currently an avid Discord user.
Why? Well the needs of some gaming communities have evolved and Discord solves those problems. I'm in various guilds in an MMO and Discord provides EASY screen sharing, EASY voice comms, lets users with privileges run small apps (typically used to allow voting on a topic, e.g. what raid shall we do, or allow simple registration, e.g. for the raid we voted on we need X tanks Y healers Z dps so click and sign up). There are fancier bots that grab in-game info and show it, letting you check on info without logging into the game, and so on.
I don't see any of this being handled well via chat rooms, forums, etc. In fact I would go so far as to be your counter-example of someone that grew up with all the old stuff and now thinks Discord is in fact a massive improvement, and not just superficially.
Meme sharing is also important since these days, the absolutely dominant way to get two kinds of info across to players quickly (good builds for your chosen character, and simple animated gifs to show fight positioning) are images. Lots of players also make build/fight video guides, but that's a longer investment in time over a simple animated gif showing where to stand and where to move. Nobody (rounded for simplification) reads text guides, in fact some info like how the group has to handle a boss fight, would be difficult to write a text guide for. You know the old saying about how a picture is worth a thousand words?
Of course it depends on those running the server and moderating, but most of the larger Discord "servers" I've joined have extensive information as pinned messages.
I did grow up with PHP forums, but they feel a bit clunky now. I'm sure a well-made one with a more modern stack and new features could be superior to Discord for text discussion though.
Many of the forum software packages you mentioned are still actively maintained and used. That's hardly a "demise."
Just because there's a wildly popular internet community in one app/medium doesn't mean that other communities simply ceased to exist. They're just the same thing they've always been - niche.
Discourse was expressly designed to re-do forums from scratch, rethinking every assumption. It's also a relatively recent creation. How is that "remaining in the past"?
The sad thing about forums is that forum software seems to have given up on itself. People clearly prefer the [old] Reddit experience over the experience provided in vBulletin, phpBB, Simple Machines Forum, Invision Power Board, Discourse, etc. These softwares remained in the past, and I think that's part of what's lead to their demise in terms of relevancy.