I've received the stackoverflow response in the past and I can say, I did not understand if it meant "this question is probably already answered on SO" or "go ask on SO because off topic" or "go ask on SO because some other reason". Because of this uncertainty, it did feel like it was meant in a rude way, but I can live with that. It basically conveys the meaning of "you made a mistake posting this question here and that annoys people", but I think in 95% of cases that is the correct use case. For people who know that SO is even less accepting of beginner-ish questions than this slack and who thought their post was not OT and who already googled, its the equvalent of "you just won't get an answer to this question". Probably an acceptable circumstance. Personally, I would prefer a simple off topic with an "OT" text as the symbol.