So I got an answer from EMO.
As long as the developer mailing-list remains used for "official" decision making, using GitHub discussions for collaboration is totally possible as it fits the openness/transparency/vendor-neutrality criteria and there is no recommendation from EMO against it. Eclipse Foundation doesn't add any SLA to what's provided by GitHub itself; so yes, we'd loose guarantee of things being backed up. But it's not a priority.
So I'm +1 for starting GitHub discussions soon and then almost immediately shutdown the tycho-user mailing-list after we send a final notice about migration to GitHub.
Committers and project leads, please cast your vote about that; on the tycho-dev mailing-list ;)