I've found slack to be a pretty poor tool for knowledge sharing like this in comparison to a more document-driven form of communication. We previously used a wiki-tool for this (I don't think it really matters which one), but in recent years we've used Basecamp for this.

I'm completely sold on decisions being driven by a doc of a few paragraphs with the conversation around it co-located in the comments. It's easy to scroll through a list of such conversations in the message board and read through the context with discussion. In comparison, finding a previous conversation is slack is much more difficult.

Even if I can find the conversation, I think that the chat-focused nature of slack encourages short, loosely worded messages. This is great in the moment but the lack of context a few months on is pretty tough to deal with.