exiftool reports the EFCS field of D810 NEFs as "Off" when the EFCS was ON. Just tried it with V9.97. The field is correctly reported by Nikon's Capture NX-D V1.2.1. For reference/debugging here is a link to two NEFs, both with MUP enabled but one with EFCS off and the other on. The following link is good for 5 days:
https://www.transferbigfiles.com/d03b3d58-cf21-4589-af99-f42a508fa8fc/pGEFcOd-b0djQsIL7FtcnA2 (https://www.transferbigfiles.com/d03b3d58-cf21-4589-af99-f42a508fa8fc/pGEFcOd-b0djQsIL7FtcnA2)
Thanks for this report and for the samples. I'll look into this, and expect to be able to resolve this for the next release (ExifTool 9.98).
- Phil