For those who do not have the flag option, looks like brave changed the naming?
If you copy in the url as described above brave://flags#enable-autofill-virtual-view-structure, it will forward to the newly named flag brave://flags/#enable-autofill-virtual-view-structure.
Then relaunch, exit and making the settings change in brave.
brave://flags#enable-autofill-virtual-view-structure
is now....
brave://flags#enable-autofill-virtual-view-structure
Also brave now has support for the address bar at the bottom (finally it's back)
enable #android-bottom-toolbar - then relaunch/exit and it's available in settings menu