It appears that google have decided that hitting the backspace is no longer valid for returning to a previous page within chrome. They claim too many people where hitting this button to delete a character but instead leaving the page altogether and losing their work. To replace this functionality in chrome they have introduced ALT + Left arrow as a way of navigating from the keyboard.
It many users its not a big deal however other will have been using this nice little hack for years so getting them to change over is a bit of a nightmare. Google have stated that if enough people complain about this latest update they will consider allowing it again (how nice of them)
In the mean time though if you are one of these people who just hate having to press the ALT + left arrow to navigate there is a nice little plugin in called backspace which can be installed.
After installing this plugin I have found it best to close all chrome windows and then the feature is back.