It's not so relevant to the conversation about memory, but is indeed a feature that has been removed.
If you want to have a single key on Linux representing going back again, I would recommend not to overload the backspace, but pick a key you don't need.
On my system F7 I never use, so it's perfect for this purpose.
$> xmodmap -e 'keycode 73 = XF86Back F7 F7 F7'
Or persistent:
$> keycode 73 = XF86Back F7 F7 F7 >> ~/.xmodmap
I hope I save you all a lot of keystrokes in the future!
