Slack is a great medium for realtime chatting but someone jumping into a conversation without context is exactly where it falls down. "Ubuntu is doomed" is exactly the sort of comment someone skimming to catch up will fixate on and jump to a conclusion - even with a code of conduct...
You need a place for conversations to exist that promotes asynchronous communication. A github issue is a nice starting point since someone can create a summary at the top. The summary can be continuously updated. And an intentionally slower/more thought out discussion can follow.
"RFC: Company$ moving away from ubuntu" is exactly the sort of productive GH issue I've seen at past companies.
Of course GH issues regularly devolve - lots of examples dropped into HN over the years where people get angry/inappropriate at some poor open source maintainer, but it is still an improvement over slack.