There seems to be more than one "YCxCz" around; documentation should reflect that.

• The two linked papers refer to Flohr 1993 YyCxCz from doi:10.1117/12.152701, a linearized CIELAB.
• In cinema there's a DCI "YCxCz" format for transmitting stereo content, probably a chroma-subsampled version of X’Y’Z’ (gamma-corrected XYZ) used in non-stereo pics. See https://dcss.dcimovies.com/0c0cff34d231b516cb89ae3fad352d5cf37a9515/dcss.html sect 4.3.2 and 10.2.3.

Yes, I came across the DCI reference to YCxCz before, but was unable to find any spec for it. Does that mean it is simply the result of feeding X'Y'Z' into an encoder expecting (Rec.709?) R'G'B'? It is not clear.

My guess is that they just use the X' and Z' as if these are chroma and just do subsampling on them. Cinema people and their secrecy doesn't exactly help with understanding.

You could well be right.

I was trying to think what appropriate weightings for X'Y'Z' would be. In theory they should be [0.0, 1.0, 0.0] since Y'CbCr luma is meant to be an approximation of gamma encoded luminance, and the Y' in X'Y'Z' already is gamma encoded luminance. But I don't think using K=[0.0, 0.0] in colour's RGB_to_YCbCr function would produce a very good encoding.