I noticed that this flag does… Nothing. ;-(
It’s not working in Beta.
It’s not working in Nightly.
(it only works in Stable build.)
So, in Beta/Nightly, we’re limited to the Vertical layout. In the illustration below (posted just as a reference), its called “Minimal”.
PS. I personally prefer Horizontal , but Chrome ditched it out of Chromium, just as they removed other useful chrome://flags that made Chrome better for me ;-( (ie. swipe left/right to change tab, bottom bar/Chrome Duet - oh, I adored it! etc.) - which is one of the less important reasons why I switched from Chrome to our, beloved Brave.