I still make it a point to write an email for any important communication that I want to document, even if I have to ping HR or the other party on slack right after.
That might be deliberate. They can use slack features to turn off saving revisions of edited messages, to make retention times short, etc. That is, discoverable evidence of any wrongdoing goes *poof* easily.
Compare to email, where you can forward some answer from HR to your personal email, with headers that later give some viable evidence you really got some specific answer, when you got it, etc. More useful than screenshots of slack, for example.
Point is that it was intended as a communication stream that is recorded and indexed, so you don't need a wiki/sharepoint/notion/etc. for example. It doesn't seem to work like in practice, though