When attempting to encode a file using the above option from the command line, FLAC 1.2.1 fails:
flac ATC.wav - works
flac -6 ATC.wav - works
flac --keep-foreign-metadata ATC.wav
- ATC.wav: ERROR reading foreign metadata: invalid WAVE file: unexpected EOF (010)
As I read the instructions page, this is the correct command line syntax.