Earlier quoted context omitted.
Most browsers except Chrome have some sort of vertical tabs support. - Safari (Mac) has a vertical tabs, but a very confusing UI, mixing Profiles, Windows and Tab Groups (only 1 level). - Edge has Workspaces and Vertical Tabs, along with Groups (only 1 level). - Chrome does not have vertical tabs and has 1 level groups - Vivaldi has vertical tabs and groups, not sure how many levels of grouping. - Firefox has Contain…
I'm surprised that none of them support tree hierarchies (like tree style tabs / sideberry), which IMO is the reason to use 'vertical tabs' in the first place.
Personally even 1-level vertical tabs are valuable because labels don’t get truncated or hidden nearly as badly as they do with traditional tabs, plus vertical scrolling is more natural and effortless than horizontal is. Additionally, most screens these days have tons of width while height is at a premium, and vertical tabs takes advantage of that.