you're still talking about _less_ information than it had before. Up until now the output was split and it seemed to be fine for IDE integration, it's been three years since "iasl4" was released. But the larger point is _any_ previous behavior could change and would have to be fixed for MaciASL; the problem with incorporating outside projects is that they may change at any time, and historical compatibility means not having a good way of breaking with previous versions of the product. The old stderr output is fully characterized, there is no chance of a false negative/positive.
Even if I added this change (which I won't), there would never be a point at which it would be safe to transition to the newer output form. I've made my objections clear in this case, that though it's not strictly exclusive, that it makes the app less explicit and safe overall. Changes may/will come later which are exclusive and will require source fixes, and that is how development will go. Only at the point of a "new" iasl, like iasl51 or iasl6, etc would it be "safe" to characterize new behavior.
It is the responsibility of users who compile their own copy to determine what the app's requirements are, and they're publicly available.