The issue is persistent since ‘Brave’ inception: Side tabs are crucial to ‘heavy’ users, and would be an eye opener for lot more sectors, which would understand how to use it if the chance would present itself - Side tabs, because of aspect ratio of modern screens is more crucial than ever both in Desktops, TVs, and mobile devices, as the first two are in ‘Landscape’ mode anyways, and lots of users would use their mobile in such a mode for more than watching cat videos on Youtube, if the opportunity presented itself.
Tree-style side tabs are not a novel idea but somehow are available only on Firefox. I would relish the chance to make ‘Brave’ my forefront Browser if only this feature was adopted, and especially if it where built-in into the code so as to create the least vulnerabilities.
Hope it comes soon, but somehow I doubt that.