Eric and I discussed this, and I've come to the conclusion that the check doesn't serve much purpose at this point.

I initially thought it conveyed useful information about the runtime behavioural restriction, but it doesn't even do that correctly, as dict subclasses (like collections.OrderedDict) will pass the check but will also be copied into a vanilla dict instance.

However, we definitely shouldn't drop it until the copying behaviour is properly documented, so I've added #17422 as an explicit dependency.