Do you even use Slack? I don't work for Slack but your comment is so misleading.
> but one thing that concerns me is that the discussion that happens is no longer (easily) searchable.
No it is not. Look at the top right and type your words. If you use free plan, only 10k message will be archived, other than that it will be deleted.[1]
> Mailing list archives are handy when you are looking for a solution to a problem. With Slack, a lot of that knowledge disappears.
Its a tool for direct communication for group, not for documentation. I think you miss a hammer for screwdriver.