Reason I am asking is it was not 3rd party in 3.9, and the codebase in question was originally compiled against that version. And it seems version included with 3.9 does not like 3.1.4(not conclusive, still testing).
Edit: Also; the 3.1.4 Prefs/Workbench has a toggle for NewIcons.
I believe the 3.9 newicon.library was a stub/emulation for programs that used newicon.library directly. You don't need it unless you have such an old program.