Thing about Discord and Reddit is it's a lot easier to moderate and control the narrative. Take a look at some of the bullshit posts on Reddit and the official forums for reference. So naturally there's going to be a drive for developers to communicate on those platforms where they can be reasonably certain that they're on home ground.
Discord for instance is not just the official CoH3 server ran by Relic though. There is dozens of bigger, CoH3 related Discord servers, even a coh2.org one. If you think the official CoH3 server has shitty moderation, then go to another server.
And idk what this has to do with Discord and Reddit having replaced older gaming forums, which is just a fact and nothing else. And a big part of this whole topic.