Commodore DOS was never formally made open source either, was it?
But if you've got a C64, you've got a license for it.
"C64 OS", you would have to buy separately. Whether it would make sense to invest time and effort into writing applications for is debatable.
Personally, as a developer who enjoys working on vintage hardware, I'd rather put that effort into either making my software not require "C64 OS" or in writing a suitable, open source replacement to that system.