This program is part of Netpbm.
icontopbm was obsoleted by sunicontopnm, introduced with Netpbm 10.53 (December 2010). sunicontopnm is backward compatible with icontopbm, plus adds additional functions, including the ability to convert a Depth=8 Sun icon, producing a PGM image.
Starting in Release 10.53, icontopbm is just an alias for sunicontopnm.
The point of the name change is that there are many kinds of icons in the world besides Sun icons, and in 2010, the Sun variety isn't even common.
In releases before 10.53, you can use the sunicontopnm documentation for icontopbm, as long as you recognize that any change the sunicontopnm manual says happened in or after Netpbm 10.53 doesn't apply to icontopbm.