It was never possible to move backwards in expansions even in retail for example. You’ll have to level a new toon in Classic.
While you’re here you could perhaps check out the Classic Hardcore launch that’s coming around later this month, if you feel that’s something for you or come back another time if you want to give WoW Retail or the Classics another spin.
Before you leave, I recommend to atleast check out the Retail Tender shop located in the Capital city to grab the free Tenders on the way out and perhaps browse the transmog. If you return you will have that atleast if something real fancy comes in.