Anecdotally, I don’t think there is a cause-and-effect correlation.
For me, Discord is a horrible, horrible, comic-book, pre-pubescent teen visual UI, and I can’t bear to use it - even for the Labs. (It also has functional limitations and is inferior to the more traditional forum-style systems like this one.)
I understand that it is free with the membership system that Relay and the Labs use, and that is probably the biggest reason it was chosen. (Hosting and running an online forum can be costly in time and resources.)
(Quick throwback - I ran an early CBBS bulletin board system on a Z80 with a single dial-in modem but stepped away when the multi-user hacks were added just before the CompuServe/American Online explosion.)
IMHO, there is a big difference between user-to-user forums and company-to-users or themed/personality forums.
My definitions:
User-to-user forums are hosted publicly by online services, Facebook, etc.
Company-to-users are primarily tech support or self-help and have migrated to Discord
themed/personality forums are started and centered on a specific topic or person/star that created it.
No affront to any of us still participating, and I get a lot of valuable help here (and hopefully have contributed some positive info in return). This forum has morphed from a themed/personality forum to a user-to-user forum.
As such, it is less successful - too small to attract the substantial user-to-user base of a large Facebook group or a Subreddit and it is rare to have a drop-in from the themed hosts that it doesn’t draw participants excited to interact with the podcast hosts directly.
There is no easy fix - nurturing an online community is a big investment in time, and the long-term soft benefits (branding, user loyalty, etc.) are hard to measure versus the real cost of information overload and time sink.