The user agent header should have been dropped a long time ago, as it is one of the largest[1] sources of browser identification data.

Even ignoring the tracking problem, using the user agent string instead of feature detection has always been terrible engineering, as it assumes all browsers within a version have the same features. Browsers have always been configurable - often in extreme ways. I don't understand how anybody can simply6 pretend the preferences dialog (and extensions, etc) doesn't exist.

[1] Out of the 15.91 bits of data that Panopticlick was able to collect from my browser, 9.0 bits (56%) were from the user agent. The 2nd largest source is HTTP_ACCEPT at 4.25 bits.