the law in the UK doesn't require any of that. It didn't even required Apple to do it. Ofcom is praising Apple for doing it even though it was not required. Social Networks need to do it.
That depends on if you live in a jurisdiction that lives or dies by free speech, and if it considers code speech[0]. Forcing you to implement age verification is effectively forcing you to speak things you don't want to say, which isn't free speech.
This UK law does not apply to OSes. It applies to online platforms. The author ran into this problem because using the iPhone required an Apple account, which could be used for something that the law applies to, but Apple didn't want to implement lazy verification and instead required verification up front.
