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.
No, email is terrible. There's no history. You can't "scroll up" to see what people were talking about. You can't link to another specific email to reference it (yes, there's archive services, but then you have to go dig through an archive to try to find a horridly formatted email to link to, which is completely separate from the normal consumption platform).
I actually argued here recently that email was hard to switch away from because of the history. You can keep your emails forever. I have emails from 2007 sitting in my inbox. And if I want to keep them, I can download them with IMAP or POP. If I send an email to five people and they remember to hit "reply all", well that's a conversation, isn't it? And if I need to show someone an email they weren't party to originally, I can forward it to them.
That puts the onus on every single person to maintain the history of the project. Lots of people delete mail (not me, but lots of people). Also, you can't retain history of what was sent before you joined. Also, emailing individual documents means people can't research by themselves.
I mean, email isn't meant to be a document repository. Slack (or other chat) is great for immediate conversations with many people. Email is great for slower conversations with a more limited number of people. A wiki is great for a very slow conversation. An actual document repository is great to be able to link to when using any of the other conversation methods.
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.
