The PNG format has chunks with types. So you can add an additional chunk with a new type and existing decoders will ignore it.
There is also some leeway for how encoding is done as long as you end up with a valid stream of bits at the end (called the bit stream format), so encoders can improve over time. This is common in video formats. I don’t know if a lossless image format would benefit much from that.
PNG is a bit unusual in that it allows a couple of alternate compressed encodings for the data that are all lossless. It is up to the encoder to choose between them (scanline by scanline, IIRC). So.this encoding algorithm leeway is implicit in a way.
There is also some leeway for how encoding is done as long as you end up with a valid stream of bits at the end (called the bit stream format), so encoders can improve over time. This is common in video formats. I don’t know if a lossless image format would benefit much from that.