In the Google support forum, Pixel/Nexus Community Manager Orrin Hancock explained that "The Pixel 2 still comes with a headphone jack but we have moved to USB-C, a standard that is becoming commonplace in the best phones and laptops of today. Moving to the USB-C audio port with Pixel 2 allows us to provide a better audio and digital experience, as we move towards a bezel-less future."
Hancock’s also listed the Made For Google headphones that have been verified to be compatible with the Pixel 2 and Pixel 2 XL.
At last he says that Other USB-C to 3.5mm adapters/splitters or USB-C headphones should work, but please be sure they are compatible with digital audio and support the proper audio standards. Thoughts?