Apple may bail on the 3.5mm headphone jack with the iPhone 7, leaving folks to either use the Lightning port or Bluetooth headphones. Apple will also probably sell Lightning-to-3.5mm headphone jack adapters that allow you to use your old headphones. As will third-party companies.
A couple of images were recently published by Macotakara and show us what these may look like. The adapters, developed by several firms, look like pretty standard third-party accessories and appear to offer support for volume controls.
Basic stuff here. These firms wouldn’t be developing the accessories if they thought Apple was going to keep the headphone jack. So just mark it off as additional evidence, if nothing else.