Lee Mather did a bunch of interviews and even David Greco (handling director for the franchise) did the one I posted in my first comment here. The reasoning for not implementing porpoising goes beyond it being an "unexpected" feature.
They didn't want to spend resources developing a feature users would probably turn off. Not that hard to imagine when you get people swearing by turning off motion blur and other visual effects for how distracting they can be.
I take you're vexed about the lack of the visual representation of the porpoising,
@tozkan7 ? That's the only thing about the porpoising that's confirmed to be ruled out. In the physics of the game you'll still get all the issues of hurting the underfloor airflow now that the plank hitbox matches the one in the car model and that we got dedicated simulation of underbody downforce. The loss won't be as severe as it is in real life by design but that's one of the main reasons we're seemingly not able to abuse kerbs anymore.
I don't like the idea of dishing out full AAA prices each year specially when the visuals are feeling stale after a couple of years. I'm all about the driving experience though and from the previews there's enough potential to pique my interest.