Seeing this purely as a spec v pragmatism issue isn't going to get us anywhere. There is just no way around looking at each case individually. There are specs that are pie in the sky, outdated, flawed compromises or reflections of vested interests. But there is also a huge amount of lock-in and lost productivity resulting from lack or disregard of specs (e.g IE6). I see no way to be principled on that one.
Which means the real debate is between two technically valid interpretations of the spec: one that arbitrarily breaks existing software for no discernible benefit✻, and one that does not.
(✻ Unless, for ideological reasons, one believes that breaking said software is the benefit, in which case this is still a sneaky and passive-aggressive way to go about it.)
It's even fair to cast this particular debate as nonsense vs. pragmatism, because that's what it is.
If someone truly believes you should break software that made bad assumptions about memcpy, as a matter of engineering principle, then just stick this at the top of memcpy and be done with it:
