This program is part of Netpbm.
pnmtoplainpnm was obsoleted in Netpbm 10.23 (July 2004) by pnmtopnm. Just use the Netpbm common option -plain.
pnmtoplainpnm exists today for backward compatibility; all it does is call pnmtopnm -plain.
pnmtoplainpnm was new in Netpbm 8.2 (March 2000) as a renaming of pnmnoraw, which was new in Pbmplus in November 1989.