Why isn't versioning being put into the spec now? If we have array.prototype.flatten why can't we have array.prototype.v1.flatten so that we have an easy out to

Why isn't versioning being put into the spec now? If we have array.prototype.flatten why can't we have array.prototype.v1.flatten so that we have an easy out to this in the future? Apis do this all the time, why can't JavaScript of backwards compatibility is this important?