The expectation that you always have to be online is also unreasonable. But if you're still required to put out a show for an unreasonable employer, you can always set it to "Busy" and disable notifications. This also signals to people that you won't reply immediately.
Of course some co-workers will want to "chat" when you answer them, but this is where you must set limits and say you'll only have time for chat outside your busy hours. This is normally perfectly acceptable IME.
My current company has _all_ of their communication on Slack, the only emails I get are meeting invitations and status emails from automated systems that haven't been integrated to Slack yet.
I can easily click to pause notifications on Slack if I don't want to get interrupted - or I can just close it and read the backlog when I have time to focus on it.
Would you use IRC? That's all slack really is.