It's in the official changelog as a new feature: http://git.videolan.org/gitweb.cgi/ffmpeg.git/?p=ffmpeg.git;...

Apologies, you're correct. I was thinking of the colorkey filter:

https://ffmpeg.org/ffmpeg-filters.html#colorkey

This was introduced in 2.8 and does the same thing as chromakey, except in RGB rather than YUV.