I think Discord is fine when you're in smaller servers. Like, the Fire Emblem Wiki server I run has about 15 active members, and it's nice at that because you can have actual discussions with people without a million other people spamming the chat at the same time. Big servers with hundreds of people in them are just a mess, though. And because the larger servers are so popular, it gets kind of annoying when you see people join a smaller server and post "hi", and if they don't get a reply within like two minutes they complain about the server being "dead" and just leave. Too many people just expect instant gratification these days.
Don't have much experience of Reddit outside of spending a little time in the FE sections. Apart from what some might call an excess of memes and fanart, the Heroes section seemed reasonable enough to me; I can't blame them too much since the game isn't the deepest thing in the world, so there's not a huge amount to discuss between updates. The general FE section was whatever, but that's probably just because in general I find myself not really fitting in with the FE fanbase (or at least, the western part of it) in terms of playstyles, opinions, and so on. I did get the impression that the up/downvote system doesn't really work out as intended in practice, though (mostly with people using downvotes to signal disagreement, rather than something contributing nothing or being offensive or so on).
I think forums like this still have their place; in areas like having more extended, in-depth discussions, and for being able to archive discussions and topics for future reference, they easily outstrip media like Discord. It's just that the popular mood is moving towards demanding more and more instantaneous gratification, which instant messaging does but forums are less suited to, and it's kind of hard for forums to do much about that. Places like this will still stick around for those of us who find them useful, I'm sure.