Don't get me wrong. I get small developers defaulting to infrastructure they don't have to set up and maintain themselves, and I understand wanting to meet people where they're at. But it's definitely a problem that GitHub plays such a central role in F/OSS development, too.
1. Git is distributed by design. Hosting on Github tends to not be controversial because that code can also live on Gitea/Sourcehut/your private git server at the same time. If Github goes down, it does not really matter. Very different from Discord, where there is no way to actually backup server/channel data, and attempting to do so may be a violation of the ToS and get you IP banned.
2. Your argument hinges on the fact that you have never seen an open-source project criticized, but it does happen. The blogpost in the parent comment even suggests not hosting on Github.
Both are proprietary websites owned by for-profit companies. Both require you to create an account in order to participate in discussion.