I find the practice more prominent here on the west coast. Having grown up in the days of two lane roads flashing lights to help passing was very common. While I have a good rear view camera I still respect the truckers that flash their lights and reply. That is what that UCC button on my smart wheel is for.:)
Much of the custom has somewhat evaporated due to changes in highways and attitudes of drivers. However, it is still a very prominent custom between Truckers on our mountain passes. It just doesn't cost anything to be nice!
Most of us who grew up in the forties and fifties generally are a little different breed than the kids on the highways today.