Yeah it would be hard to define, no doubt, although with Apple's limitless resources and huge power, I think they need to at least try. The current subjective whims are clearly not good enough and they've gotten so powerful that human judgment isn't acceptable. I would say the same about Twitter/Facebook/Youtube/etc.

I suspect the hardest part would be defining it in such a way as to disallow Parler but still allow Twitter et al. Which, kind of proves the point of people who are suggesting Parler is being held to a different standard than the others.

IIWM (which of course it is not) I would start with something like, "Platform has a system in place to provide early detection of explicitly violent content at a rate of at least ${x}%. Platform has a system in place to allow users to report content in violation of the platform, and review of reports is completed within ${y} hours."

Of course what is "violent" is subjective too. To some people talking about hanging a person is not violent, while to others calling someone by the wrong pronouns is violence. A common standard would indeed be hard, but I think it is important.