Out of curiosity, why did Opera not disable this change?
Because it wouldn't make sense? If they would be changing Chromium all the time, there would be no meaning in using it.
There are many unhappy campers because of this.
Yes it can be resolved in Settings, but why annoy so many users?
Well, the key was disabled because the previous behaviour was annoying many users. So i guess the other side won this time.
Basic users don't even know about these forums or settings. They just want a browser that goes back a page when they press the Backspace key.
Average users most probably don't even use the keyboard to navigate.