User-Agent: Chromium/47.0 (Windows 10)
User-Agent: Firefox/43.0 (GNU/Linux; X11)
User-Agent: Wget/1.15.4 (GNU/Linux)
etc.
Given that current UA strings are abysmal (and sent on every request), this would probably save a few kilobytes worth of headers (images, scripts, stylesheets), so would probably improve performance by some nanoseconds.
They managed to do this with SHA-1 deprecation and a lot of other stuff. So they could just announce that "on February 1st, 2017 we all clean up the clusterfuck User-Agent headers are", bake in the timer and spread the word.