The only backwards compatibility breaking change I know of is that function declarations are scoped block level in EcmaScript 6 (which I think has been renamed

The only backwards compatibility breaking change I know of is that function declarations are scoped block level in EcmaScript 6 (which I think has been renamed to EcmaScript 2015), not function scope which is what they have been since the beginning, but they're only block level if you use "use strict"; for the foreseeable future and if this change breaks things for you you were writing really terrible JavaScript to begin with.