I have been using computers for 40 years and I cannot see a single feature of discord that I cannot have on a forum or even reddit. In fact, the UX of Discord is abismal.
Of course, this isn’t always the case, your messages can still go unanswered for days. But in my experience, when I see that a project has a Discord, I now think "Great I might get an answer". And this doesn't dismiss all the cons that others have already mention
Look at the VIM help as an example of great manual. Or the GNU software documentation. Now, you see “Join our Discord” instead.
The UX on Discord is excellent. That's why so many people, including nerds that used to happily use IRC and Mumble, use it. It's easy to create your own server, administer it with granular permissions, and to invite your friends to it. You can instantly start voice and video calls with screen sharing, for free. It's also a live chat with excellent formatting capabilities and has multiline support, unlike IRC (unless this has changed recently?), which makes it much better suited for technical questions involving code. Oh, it also has actual code formatting built in so it's actually legible (though this remains a feature underutilized).
The problem with discord is it's just not visible outside of discord, it can't be indexed, archived and searched through other means, and is vulnerable to the whims of the server owner. It's also not very good at being a forum replacement because its threading capabilities and UI wasn't designed for that purpose, so it feels bolted on (because it is).
[0] hacks aside
People on this site really confuse this issue: your dislike of Discord is an ethical one, because it goes against what you believe a chat should be (free/open/whatever). Most people unfortunately just don’t give a shit.
As far as I know there is no other voice communication program that handles voice channels TeamSpeak style with a chat box next to it. If there is one I'd jump ship.