This program is part of Netpbm.
ppmquantall was obsoleted by pnmquantall, introduced with Netpbm 10.58 (March 2012). pnmquantall is just a renaming to reflect the fact that the program handles all kinds of Netpbm input files, not just PPM. Though this was not always the case, it had been for many years before the renaming.
For Netpbm before 10.58, where the program is still named ppmquantall, just use the pnmquantall manual.