I think that many times, you only bring things to list after they reach a certain level of maturity. Sometimes on slack etc you are just working out how you feel about something, trying to understand something. I think it is perfectly fine to have such conversations on slack.
Perhaps you find that there is consensus but you want to inform folks on dev list. Then you can make a lazy consensus thread. Perhaps you find that there is disagreement and you need community input to help resolve. Then you can make a discuss thread. Or perhaps you realize that there is more or less agreement (or disagreement but acceptance) and there is no need to bring it to list. All such outcomes are possible.
