Well slack doesn't even send me the messages to my desktop continuing to send to my phone (which continues to buzz) despite me writing on the desktop, just because I send a message on my phone 10min ago.

So it might not be that easy (if even slack with their funding can't get it right).

It's "good enough", especially if the alternative is my phone not vibrating when I get a message and I'm not at my desktop.

Yes, I don't want any clever tricks from my chat clients. I dislike this kind of under-the-hood magic. Client should just receive the message and send a notification if it's not silenced.

Well, it's not great, but could be a viable solution until a scheme is devised that is guaranteed not to lose any notifications, which would be worse.