(Replying to parent and GP)
I wasn't suggesting an IM system should be the source of truth for long-form documentation. But searching Slack works better than searching email you weren't cc'd on. Virtually anything is better than email for many essential aspects of collaboration.
Want to add someone to the mailing list and loop them in?
How many prior emails will you forward to them?
Someone replies to one of several points in an email, someone else wants to address a different point, and there are a mix of habits and patterns for whether / how much / where to quote or include the previous messages in a response....
vs adding someone to a channel where they can review the conversation history, and some combination of threads and links to additional channels or external resources can adequately support whatever direction(s) the conversation takes.
In any case, it's the wrong tool for the job in so many ways, so much of the time. Hence my recommendation to keep emails to 5 sentences, and to prefer communication tools that properly support multiuser collaboration.