The argument was that most of the mentioned problems could be solved by quoting the values. I don't have a problem with avoiding "on" as a key, and I apparently haven't used it ever, because I've never run into this particular problem in my 15+ years using YAML.
So, sure, if you want to play it super safe, quote keys as well. But I'm personally fine with the trade-off in not quoting keys.
If you compare to JSON5 instead of JSON, you still get the benefit of unquoted keys, but you also get a guarantee the keys are strings, and it's harder to forget to quote a value.
So, sure, if you want to play it super safe, quote keys as well. But I'm personally fine with the trade-off in not quoting keys.