Ditto, i looked around and i still can't find any official reasons why the rumble was removed. There's plenty of speculation such as being unable to find the parts, or being too expensive and they needed to cut something from the budget. Maybe they figured it had low demand.
But i'm also seeing stuff about the controller board being changed so a person couldn't add rumble even if they had the rumble motors.
My guess was that the rumble was interfering with something. But again, it's just speculation.
I really wish the GPD People will give us an official statement as to why it was removed and if they'll ever release a kit for people to add it back.