Right you are, I had forgotten about OmniWeb. But indeed, it has very nice vertical tab support. Not really an option for modern Web browsing, but Omni does keep updating it in small ways. The test build page was last revised on 12 April 2023, and that version launched fine on my iMac running Ventura.
Iām using Vivaldi on Linux (sorry, folks). I turned on vertical tabs, and I like them. However, some websites seem to break: the left sidebar disappears, seemingly replace by the vertical tab panel. I discovered that the website sidebar reappears when I make the browser window wider. BTW, the disappearing sidebar act occurs whether the vertical tabs are on the left or the right.