I use Backspace back quite a bit myself and was confused when it stopped working but I think it's the right decision to remove it. I'll just switch to Alt-Left rather than switch to an extension (and endure the week or two of annoyance while I adapt).

I actually use Page-Down instead of space which I suspect makes me weird but I feel like Space should be next on the hit-list as long as they are cleaning up confusing shortcuts.