Structure has changed for custom dimensions, but something appears to have changed in dimension type files and is not recorded in a change log. When I use the new folder structure I get the following error:
WorldGenSettings: Failed to parse dimension_type:dimension_type/default.json file: dimension_type:dimension_type/default.json
Several friends have experienced this as well for their datapacks. I have seen similar tickets, but they did not include this specific error.
I have attached a datapack that works in 20w30a. You may restructure it yourself to ensure that is not my error. Once restructured you should get the error.
Several changes were made in pre-1, you have to update your pack:
https://www.minecraft.net/en-us/article/minecraft-1-16-2-pre-release-1