I have a long term creative flat world that I created during 1.16. Since 1.18 I have built lots of structures underneath the y=0/bedrock layer. When I updated my game to 1.19 and loaded the world, I found that most of the things I had built below y=0 had been deleted. A few chunks of area below y=0 were not deleted, but most were. There were lots of things built in the area below y=0 in the following image, most of it now being gone.