They were only useful for obtaining compatibility for old applications
that used libpng-1.0.x versions prior to the implementation of "free_me".

except in pngconf.h where they can be used to request that
PNG_FEATURE_SUPPORTED not be defined.

in png.c (suggested by Peter Breitenlohner)

to see if that works better for the auto-license-detectors

    to "#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED", to make the png_set_keep
    mechanism available for ignoring known chunks even when not saving
    unknown chunks.