Technically no. I read up on the spec a while back. GET parameters are probably better referred to as URL args, and they're always retrievable via the URL in the first line of the HTTP request. In the case of a POST, the content of the request may (also) have arguments.
It's not super common to use both, but it's definitely not disallowed by the spec.
I'd be interested in seeing any references about this. From what I've read, it seems the intent is to not combine them, but I haven't seen anything explicitly forbidding it. Do you have a source that indicates combining URL parameters in a POST is encouraged, or at least permitted?
"I noticed that in the case of POST handle-request-thread (srv.arc) throws away the GET args."
Which seems to be criticizing what I would consider perfectly valid behavior. I'm not sure why you would want to split your parameters for a POST between the POST parameters and pieces of the URL.
If a change to the source is being suggested, it seems reasonable to determine if it's warranted by the protocol.