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.