Out of the question is the term in use today. It means beyond consideration;
unthinkable or impossible (Collins dictionary).
Out of question is obsolete, which means no longer used. When it was used, it
had two meanings.
The first one was unquestionably or doubtless. See Definition 5 in Universal
Dictionary of the English Language. Today, for this meaning we use without
question.
As you can see from Definition 6 from the same dictionary, it also meant out
of the question or not to be thought of. So it was just a variation of the
same phrase with the definite article. (As an aside, this shows how idiomatic
the use of the definite article can be.)
The Oxford English Dictionary gives an example of out of question, as a
variation of out of the question, as recently as 1940:
Flight tests have shown that more than one glider can be towed by an
aeroplane, so that a glider train is not out of question.
(from the magazine Aeronautics).
So, in general, you won't see out of question in use nowadays, but in older
uses (that are now obsolete), it could mean either the same thing as out of
the question or it could mean unquestionably.
https://ell.stackexchange.com/questions/95017/
the-meaning-of-out-of-question-vs-out-of-the-question