> Call it github-flavored YAML (GFY) or something and accept a different file extension.

Sure, I wouldn't be upset if they did this.

To be clear: there aren't many fully conforming YAML 1.1 and 1.2 parsers out there: virtually all YAML parsers accept some subset of one or the other (sometimes a subset of both), and virtually all of them emit the JSON object model instead of the internal YAML one.