The feature has not made it to the stable version yet, but the beta version just got this working fairly well. With many desktop screens being 16:9 ratio it makes a lot of sense to use the space on the side for tabs. I know Vivaldi and Edge have both had this for a while, but I left Vivaldi after seeing they were not contributing their improvements back as open source code.
Brave's is not perfect yet as it defaults to collapsed view and there is no adjustable expansion yet. But it looks like that is going to looked at.
Just note, especially if you have tabbed groups, that after enabling this with brave://flags/#brave-vertical-tabs some may see only one tab. If so, you may have to disable/default the tab scrolling with brave://flags/#scrollable-tabstrip.
So early days still, but I'm glad it has finally appeared and is working.