-s (silent) is also undocumented. There’s a confusing distinction between it and -q (quiet), where the latter isn’t quiet in the usual sense; -s is needed to silence non-error output entirely. There’s useless output even when called entirely without options:

    $ pngcrush 
    CPU time decode 0.000000, encode 0.000000, other 0.000000, total 0.000002 sec
    $ pngcrush -q
    CPU time decode 0.000000, encode 0.000000, other 0.000000, total 0.000002 sec
    $ pngcrush -s
    $

This is pngcrush 1.8.13-0.1 on focal.