That's a good point. Email is much worse than slack.
But I still think it is impossible to manage all the things happening in slack. And the expectation is that it was said in slack, so you should know about it! Whereas, I definitely go through and review PRs and if there is a culture and management agreement around good PRs, then I can easily understand how things work, how things have changed, etc. I never get that from slack. And, I never got it from email either, fairly.
The topic of "expectation" is really interesting and worth discussing more. I refuse to accept the expectation of slack meaning I am literally always available to message, and also that I must read and keep track of everything in there. If somebody asks me about something and I suspect it might be in slack, I might do search for it to catch up on that topic if I am inclined. But obviously so many people feel the pressure you're describing (to know about everything said in slack)--- which sounds terrible!
But I still think it is impossible to manage all the things happening in slack. And the expectation is that it was said in slack, so you should know about it! Whereas, I definitely go through and review PRs and if there is a culture and management agreement around good PRs, then I can easily understand how things work, how things have changed, etc. I never get that from slack. And, I never got it from email either, fairly.