In practice much bigger practical problem is that no-one handles HEAD.

I sure don't.

And those that do probably aren't careful about HTTP headers either.

So in practice this an improvement for most.

And if you don't like how it works, there are plenty of alternatives to use.

How does it go against the HTTP Spec. The HEAD method could be autoregistered for all handlers of GET Requests and library makes sure no body is sent (Basically it's a autoregistered middleware that wraps the get handler and overwrites the reponse writer with an empty body.)

Arguably this is more correct that letting the users declare a separate Handler that can neither guarantee the same headers as the the GET Handler not guarantee that the body is not sent the response.

Go implicitly buffers the first 512 bytes to sniff a content-type header so it's no like the status quo is free of arguably incorrect magic.