AFAICT they moved off IRC in response to Mozilla's announcement that it was being deprecated and also because IRC was terrible. They tried Zulip but concluded it wouldn't work well. There was already a community Discord server doing well so they set up an official one.
I suspect they might move back to Mozilla's Riot/Matrix instance once it's set up in production, but that hasn't happened yet.
I suspect they might move back to Mozilla's Riot/Matrix instance once it's set up in production, but that hasn't happened yet.