Why? and why can't I choose if I like my neighbours to stay put. I would just of soon have 20 players that trade in 50,000 units than 200 that trade in hundreds.
You never had this option, the only option that ever was available was to freeze yourself in your spot, it had zero effect on your neighbors and simply means that you will be left alone in an empty wasteland when all your neighbors move away in the next way of moving cities.
The option was removed simply to avoid zillions of complaints by people that are left behind and suddenly would have preferred to move along with their neighbours, but they opted out not understanding what they opted out of.
It is far better without this silly opt out option. It would have been better if they had never invented it in the first place, then nobody would ever have missed it. Makes the process much simpler, faster and easier with a better result for more people.
Give the people the option to shoot themselves in the foot and they will take it...