This program is part of Netpbm.
pnmcut was obsoleted by pamcut, introduced with Netpbm 9.20 (May 2001). pamcut is backward compatible with pnmcut, plus adds many additional functions, including the ability to process PAM images.
pnmcut remained in the Netpbm package until Netpbm 10.46 (March 2009) because of hopes that it had fewer bugs than pamcut because of its age. But now it would just be clutter.
In Netpbm before 9.20, use the manual for pamcut with pnmcut. Features that are in pamcut but not pnmcut are indicated by statements that they didn't exist before 9.20.