Default state: most likely on
When it’s on: vertical tabs behave like they did before the more recent RCs.
When it’s off: vertical tabs have that show-on-mouse-hover behaviour
Shortcut: toggles between off and on
Menu item: either under “show toolbar in full screen” or “hide tab bar”, since it seems similar to either.
It could go under the name “Always show vertical tabs in full screen”, or just simply “Always show vertical tabs” if this feature is meant to exist outside of full screen too, which it currently sort of does.
There’s another option, you could just add that behaviour to the “hide tab bar” menu item, which is currently disabled in vertical tab mode and has no shortcut. Name fits as well.