In my team we use free version of Slack with 10k messages archive. Surprisingly, this is something that helped us overcome the problems you are mentioning. People got angry when they needed to ask the same question again and again because the old ones disappeared. Pinning items down created a mess of documents, answers and snippets. We unanimously decided that we need to move things to wiki, no excuses. Now Slack is just a stream where we discuss things. After making decision we move the info to the wiki or documentation and it works well. Thank you for the post as it will remind us all that Slack is just a tool, and the responsibility on learning how to use it is on us.