You can no longer disable material design in Chrome’s browser starting from version 55. Recently Google has made material design the default UI for Chrome – first on Mac with version 52 and next on Windows for Chrome 53. Users who’ve not liked the new design can disable Material design and revert to old UI temporarily by using the Material Design in the browser’s top Chrome flag, which had not lasted for long, Chromium team has removed it.
Material Design in the browser’s top Chrome flag removed
This change has been spotted on Chrome dev, expect it to roll out to Chrome 55 stable, which could arrive in a couple of months from now. We advise you to get used to the new design rather than blaming Google or stick with older versions of Chrome instead, which could be a bad decision as you not only miss the new features, but also the security fixes as well.
Thanks to Hamuko.