I was using Slack a few years ago and I didn't have this issue. Everyone who chose to use it had IRC experience, and so immediately treated it as a fancier IRC. That includes a lack of expecting an immediate response.

But I can definitely see how people who didn't grow up on freenode et al might come in with different expectations around Slack responsiveness.

for me slack is an async function. its on silent, the entire time. And my time slices are large (2±1 hours)

I try to do this too but Slack updates the favicon when in do not disturb. It’s like catnip, plenty of dark patterns

There's an option in the Preferences to "Show a badge on Slack's icon to indicate new activity" which, when unchecked, may stop the behaviour you're describing.