In practice, people expect a response in a relatively timely way, it is after all chat, not email. It does seem a lot of companies treat it explicitly as ephemeral (and therefore requiring frequent checks). Even slack deletes old messages for free accounts, and the search is pretty poor IME. It is also full of chatter irrelevant to the topic at hand as far as I've seen in the slack groups I've participated in (though I have avoided it for work, so have not experienced it working well in a work environment).

I can imagine it is very useful for responding to a live incident for example, so perhaps people use it for that sort of thing. For a developer working on product though it seems crazy to me to have a chat window open for any significant part of a working day. People work differently though I guess and perhaps I just haven't seen it used well.