What you're looking for is email. Multi-user chat simulates a real life conversation, and in real conversations it's also awkward to say "hey so going back to what we were talking about 15 minutes ago..." after the conversation has moved on.
Just because Slack isn't good at slow conversations doesn't mean it's bad. Just because email is bad at including people who joined after the fact doesn't mean it's bad. Those aren't their strengths, and that's fine. They still serve their own purpose just fine.