@leocg the need for vertical tabs is there. I don't think it's a fair statement to say that every mainstream browser in the market today implemented this useless feature. There is a very clear need for it. To say that "well it wasn't popular 20 years ago" completely ignores the changing landscape of technology and how fast needs arise.
Opera air is supposed to be a productivity browser designed to help focus. Vertical tabs help tremendously with the workflow. Being able to in a list form every tab I have open and jump too it is massive. Also being able to group tabs by like content is also a game changer.
Here's a great example. Why do we write lists vertically and not horizontally? Tabs are nothing more than a list of all the open items. Sure if you not doing much then a horizontal list works but once you have more than a few things it makes sense to list it out vertically.
I even see this in my org. I'm an admin and manage thousands of users. Browsers settings are completely open for end users customize as they please. Most users have either moved to edge or chrome and use vertical tabs.
Also I've seen people using vertical tabs on chrome not sure if it's an extension or built in as I'm not a chrome user. Edge and opera for me but mostly edge since opera lacks these features