Yes, my mistake. I assumed common sense.

https://pico-8.fandom.com/wiki/P8PNGFileFormat

Actual cases of proprietary chunks include iDOT from Apple (apparently a performance optimization for plain images)

https://www.hackerfactor.com/blog/index.php?/archives/895-Co...

and the Macromedia Fireworks save files

https://stackoverflow.com/questions/4242402/the-fireworks-pn...