Please stop making every update require more screen space. Every time, more chrome, bigger fonts, more padding, more views that take over the whole viewport.
The other side here is that when I've designed apps that more dense, smaller font sizes and UI, people do then complain things are too small. Engineers are generally ok and appreciate density but more mainstream users often don't.
If they exposed an api or used an open protocol, power users could build their own app for slack instead.
I really hate this change too, on my laptop, slack has to be full screen to make sense whereas I can have four screens of terminals and email without issue otherwise. Wish they at least made the design adaptive