While I agree with the concerns voiced here, I think there are two things worth pointing out:

1. Some of the issues you list here are due to how novel this new way to communicate is (asyncronish ✨). That doesn't mean it's inherently bad, though of course it has downsides, it just means a new etiquette should be created for it — and eventually, it will. Slack has a heavy hand on the direction this will take (do not disturb, alerting when you'll notify too many people), but ultimately it's up to us to push our communication habits in a sensible direction. Which leads to an even more important point:
2. Many of the aspects you mentioned are not necessarily a reflection of an unhealthy communication system, but a reflection of an unhealthy workplace/collaboration culture. It's not only Slack that's implying you are (or should be) always on, it's you and your colleagues who are subtlety enforcing this culture. Sure, you may get too many messages from 10+ slack channels, but doesn't that raise the question that maybe that's too many channels to be on? That maybe some people should just email you? Maybe synchronous conversations should be a privilege, not a given— you certainly don't give your iMessage number so random people on the internet can contact you anytime, so why should they do it through a public Slack channel?

The points here are 100% valid and push the discussion forward (and Slack should have these questions in mind when thinking their product in the long run), but it's also our responsibility to question this state of affairs that implies we should work and live in a way that we really don't want to. Congrats on the article, as it does just that!