“It looks like nothing has happened with Apple’s code since a security fix in 2022, and the repo comment makes it sound like Apple is intentionally leaving its support stuck at an old version:
Apple CUPS is the version of CUPS that is shipped with macOS and iOS. For the current version of CUPS that is used on other operating systems, see https://openprinting.github.io/cups for details.”
The archiving of the repo seems to imply they fully moved to https://en.wikipedia.org/wiki/Internet_Printing_Protocol / https://www.pwg.org/ipp/everywhere.html in their next OS release (might be based on CUPS, but also could be a clean sheet implementation)