If the language spec requires TCO, I think you can reasonably call it part of the language.

It wouldn't be the first time the specs have gone too far and beyond their perimeter.

C's "register" variables used to have the same issue, and even "inline" has been downgraded to a mere hint for the compiler (which can ignore it and still be a C compiler).

	▲	201984 an hour ago \| parent [-]
		inline and register still have semantic requirements that are not just hints. Taking the address of a register variable is illegal, and inline allows a function to be defined in multiple .c files without errors.