I don't think they will. Apple would be contradictory to retain the headphone jack on an iPhone model if it's re-introduced and they would rather have uniformity across the whole iPhone line up. Especially given the AirPods to continue to sell with the 3.5 Jack being deleted.
That’s what I would have thought of five years ago but today ? I wouldn’t bet on it.
Apple have completely lost their uniformity, their line up is a mess right now :
-For some reason every iPads they offer still have a headphone jack while the newer iPhones doesn’t.
-Every MacBooks still have a headphone jack too even though some have gone full USB-C (which can replace the headphone jack by the way).
-Also don’t forget the newer iMacs that still have USB-A and SD card slot even though they also have USB-C (which again can replace both of these older ports).
-Let’s not forget the mess of their color choices too with a missing rose gold on the iPad Pro 12.9 versus the 10.5, a missing gold color on the iPhone X versus the iPhone 8 and 50 freaking shades of space grey across devices.
So yeah, needless to say that Apple have been unpredictable these days...