While forums are outdated, I haven't seen anything new come along that serves the same basic function, but better. IE, leaving a message or question, then coming back and having all the replies to it in a nice, clean, simple interface. I mean, social media can do it, but in a worse way since they tend to adjust the order of replies and split things off. Discord can do it now with threads, but most servers don't use that and for those that do, they tend to not be organized well. Ironically, this game's official discord server is one of the best I've seen for using threads in it.
Forums are bad if you're looking for a very quick, immediate answer to a question, sure. But if you're looking for a discussion, or if you're just posting a question say, while on the toilet getting ready for work, only needing the answer when you get back from work, forums are vastly superior than 99% of discord servers are.