I know Stardock used to use IRC around 15-20 years ago, so I don’t think it’s anything new for older devs.
Newer devs want the control Discord allows, you get fewer spammers/trolls on Discord. You also can shut down revolts a lot easier.
I’m not on any official game discords: it’s mostly a webcomic, local community, FGC community , and calibur character discords.