Who can and should make changes to POH required to be carried in a 1979 model Aircraft. My question is especially directed at section 9 supplements. The most of the equipment listed in section 9 of the aircraft I fly is no longer installed in the aircraft and new equipment installed but not covered in the supplements. Manuals for the equipment are carried with the aircraft.
If that new equipment came with an AFM supplement (which happens with pretty much all new avionics), the AFMS should be added to Section 9 of the POH/AFM. This should have been done by the installer, but as PIC, you are responsible to ensure it's been done. You can remove old AFMS's for equipment which has been removed, but you don't have to.
I have a three-ring binder for each aircraft I own. I keep it up-to-date with new equipment manuals/info and remove old manuals for retired components.