This program is part of Netpbm.
pnmnoraw was replaced in Netpbm 8.2 (March 2000) by pnmtoplainpnm, which was obsoleted by pnmtopnm in Netpbm 10.23 (July 2004).
pnmtoplainpnm was actually the same program; it was just renamed to make it clear that is just a format converter.
pnmtopnm is more general, in that it can go both directions. pnmtopnm -plain is the same as pnmnoraw.