In ECMAScript 5,
yield is a strict-mode "Future Reserved Word":
126.96.36.199 Future Reserved Words
The following words are used as keywords in proposed extensions and are therefore reserved to allow for the possibility of future adoption of those extensions.
class enum extends super
const export import
The following tokens are also considered to be FutureReservedWords when they occur within strict mode code (see 10.1.1). The occurrence of any of these tokens within strict mode code in any context where the occurrence of a FutureReservedWord would produce an error must also produce an equivalent error:
implements let private public yield
interface package protected static
This means that in non-strict code, your concerns are correct: the correctness of the line
var yield = 5; will differ in non-strict mode between an ES5 and ES6 implementation. However, in strict mode, the line is invariably invalid for both ES5 and ES6. This means that you should use strict mode whenever possible to maximize forward compatibility of your ES5 code.
As for why yield was not fully reserved for non-strict code, I assume this was done to bridge compatibility between ES3, which did not include
yield as a FutureReservedWord (see ES3 section 7.5.3, on page 14). In order to introduce
yield, the commitee had to break compatibility somewhere, either in the 3-to-5 transition or the 5-to-6 transition. However, some reserved words that were included in ES3 were downgraded to strict-mode reserved words in ES5. I do not know the rationale for that decision.