Turning your head ~90 degrees to the left just to be able to see Slack(or any other windows) is for me an productivity nerf, not a boost. What's wrong with Alt/Option - Tab instead? In my life I experimented with 3 monitors, 2 monitors, one monitor, ultra widescreens, square aspect ratio displays, and a mix of several of those, etc. and I find the best productivity setup is when I have everything centered in my narro…
I am sure a setup as shown in the video could help someone like a stock trader as they are used to getting large amount of input from multiple sources. But I think for most people, to actually get things done, they only have this much attention and can do limited amount of multitasking. Having to arrange windows is also a real hassle. And for me, I already use workspace features for different projects, which is a bit burdening, and having infinite amount of windows around is not going to help.
Which is why I never bothered to add a third monitor when I could easily do so. I tried it out a few times with my tablet but found that display mostly unused.