I just exiftool-ed a Z6 NEF, and the “WB RB Levels” makernote tag is still the same ID as my D7000. You might get exiftool and run this:
exiftool -G -H yourfile.NEF
and look for something like:
[MakerNotes] 0x000c WB RB Levels : 1.482421875 1.482421875 1 1
in the prodigious output.
Nikon Z NEFs have new stuff in them, e.g., I’m staring at an XMP section with editing parameters I’ve not seen before, but the WB Levels tag hasn’t changed…