Why MOV AH,1 is not supported in 64 bit mode of intel microprocessor?

In the book "THE INTEL MICROPROCESSORS" of Barry B. Brey, it is written that

MOV AH, 1

is not allowed in 64 bit mode, but allowed in 32 bit or 16 bit mode. If MOV AL, 1 can be allowed in 64 bit mode, what is the problem with MOV AH, 1 ?