As far as I know, pageXOffset/pageYOffset properties were already available since Netscape 4 era.
And it seems scrollX/scrollY were introduced circa Netscape 6.
Alternative question:
Q2. Is there a browser which implements scrollX/scrollY but doesn't support pageXOffset/pageYOffset?
I will add a third question because no one was able to answer the previous ones:
Q3. scrollX/scrollY was added to the latest editor's draft of the CCSOM and the working draft only got pageXOffset/pageYOffset, why are they keeping both attributes?