It has fields to say what compression is used. Adding another compression form should be handled by existing software as recognizing it as a valid PNG that they can't decompress.
The PNG format is specifically designed to allow software to read the parts they can understand and to leave the parts they cannot. Having an extensible format and electing never to extend it seems pointless.
The PNG format is specifically designed to allow software to read the parts they can understand and to leave the parts they cannot. Having an extensible format and electing never to extend it seems pointless.