us.map from kbd maps Right Alt to AltGr and Shift+Tab to Alt+Tab.

* I have never seen a modern US layout keyboard that has AltGr at the
position of Right Alt. Also, using emacs ergonomically requires using
two alt keys. Sometimes, I use emacs on linux console.
* In us.map, Shift+Tab is mapped to Alt+Tab. I see no good reason to
map Shift+Tab to Alt+Tab unless the keymap was neglected after someone
tested it with an esoteric keyboard or in an esoteric environment.

For now, I use my own keymap that inherits us.map and
linux-with-two-alt-keys and maps shift+tab to shift+tab and alt+tab to
alt+tab.

I propose mapping Right Alt to Alt and Shift+Tab to Shift+Tab in
us.map or a modern variant of us.map.