I noticed @Vlad that you mention a lot that this is not according to Apple's guidelines. However, this should not mean that the feature request / UX is bad. It seems that community would appreciate such feature, meaning that it is a good UX. I learned about it in Arc Browser, which is why I really like it. Their vertical tab management is far more superior than any other browser out there (Favourites, Pinned / Folders, and auto-closing tabs the rest) with the addition of auto-closing sidebar.
Besides, those who do not like that, can always click the toggle button on sidebar to permanently PIN it there.
I agree with @Nitrousoxide wholeheartedly, Focus mode is not even close to what we want. We want to utilise the real estate of the browser to view the content, not the tabs, and then easier switching between tabs. In fullscreen mode, you still have to click the tab menu button to bring back the tabs. Why not just hovering over the edge instead?