I have never had any issue finding information in slack with history going back nearly a decade. The only issue I have with Slack is a people problem where most communication is siloed in private channels and DMs.
Email threads are incredibly hard to follow though. The UX is rough and it shows.
The fact that there's a subject header alone leads people to both stay on topic and have better thought out messages.
I agree that email threads could have better UX. Part of that is the clients insistence on appending the previous message to every reply. This is completely optional though and should probably be turned off by default for simple replies.
Email is really powerful but people simply aren't good at taking advantage of it and it varies by email client. Doing some IT work at a startup made this pretty clear to me. I found Slack was much more intuitive for people.
Both systems rely on the savviness of the users for the best experience and I just think email is losing the UX war. Given how terrible people seem to be at communicating I think it's a pretty important factor to consider.
It doesn't help that Outlook's search capabilities have gotten effectively useless - I can type in search terms that I'm literally looking at in my inbox and have it return no results, or have it return dozens of hits without the search terms involved at all. I don't have that problem with Slack or Teams.
However, I think you are right overall on email being better overall for what people end up using chat apps for.
People love complaining about the email workflow of git, but it is demonstrably better than any chat program for what it is doing.
I'm thinking of things that are assembled. The correspondence that went into the assembly is largely of historical interest, but not necessarily one of current use.
