Linux

The counterpoint is since 16:9 became the de facto standard for monitors, vertical resolution is at much more of a premium than horizontal resolution is.

Try out Tree Style Tabs for an hour. I'm curious how you'll feel about it.

since most languages are written horizontally and i like ux to reflect this structure. such things are subjective though

You might be misunderstanding what we mean by vertical tabs - we aren't literally turning the tabs sideways and putting them on the side of the browser. We're placing the tabs, still horizontal, into a stacked, scrollable list on the side of the browser. The superiority of this display method for tabs on widescreen displays is not subjective, and here's why:

1. Tab titles are not typically very long, but there tend to be a lot of them. This data is far more readable and accessible as a bulleted list than a long paragraph.
2. Beyond about ten to fifteen tabs, tabs displayed at the top, side by side, must either shrink and obscure the title, go off-screen and be invisible without scrolling, or stack in multiple rows across the top. A vertical tab setup can easily display 30-40 of them in a vertical list, all with the maximum visible amount of their titles which helps distinguish them from one another.
3. Modern desktop screens are wider than they are high, but webpage content scrolls vertically, often leaving a lot of empty space on the sides.
4. Eyestrain is reduced and readability improves when the width of the reading area is reduced. This is why text on the web almost never fills the full width of a widescreen display, why most books are taller than they are wide, and why newsprint articles have many narrow columns rather than filling the entire page.
5. Given points 3 and 4, tabs at the top of the browser window on a widescreen display leave slightly less room for the actual page contents, while tabs displayed in a vertical list on one side only cut into the white space that exists on the sides of the content, while keeping the titles readable and causing less eyestrain.
6. With one change, a list can become an outline with sections and headers, following your own train of thought as you branch out and expand on each idea. In the same way, tabs displayed as a list can be very easily displayed with a tree structure, allowing tabs to be grouped, collapsed, and generally organized in ways that are impossible for traditional-style top-tabs.

This is why Tree Style Tabs exists, though I prefer Sidebery these days, being more customizable and performant than TST. There's no way I can ever go back to top-tabs.

I think you're missing what's going on. The text is still written left-to-right. You don't need to read the tabs vertically. The tabs are stacked on top of each other in the sidebar instead of lined up along the top of the window.