While Apple did not announce removal of headphone jack during the presentation, it is a fact that Apple has quietly removed it. The new iPad Pro makes use of UBC-C port instead of lightning and now, just like iPhone, you will have to use an adapter to use your old 3.5mm headphones with the iPad Pro 2018.
However, unlike iPhone that has lightning port, you will have to use USB-C to 3.5mm headphone jack instead that goes for nine dollars.
Apple clearly wants its users to move to wireless bluetooth headphones but not all users are happy with this decision since iPad Pro is a pretty large device and there is plenty of space to fit it with a headphone jack.
On the bright side, you can use the adapter with the Macbook as well that too have the USB-C port.
So if you wanted to use your existing peripherals that make use of 3.5mm headphone jack, you will have to buy this adapter and carry it everywhere with you. More dongles for you!