I definitely see an issue with this. Fortunately, at least GitHub is readable without running non-free code and is searchable. I guess you can also participate with one of those alternative clients.
You are still not in control of your community, and I'd like this monopoly to vanish.
Not really. GitHub is also "readable" through a search engine, and can be used (read-only) without signing in. GitHub also allows you to use automated tools to read and manipulate the data stored there. Discord does not allow you to do any of those things.
It is more of the content inside GitHub is widely accessible without needing to log in whereas Discord are not really accessible and often would need "invitation link" to find those servers. GitHub is far more public in this sense than Discord is. Discord/Slack is a instant message system, far different, than forums style which are "static", which make archival extremely difficult to do in Discord/Slack.
You are still not in control of your community, and I'd like this monopoly to vanish.