Loading worlds from previous releases with custom dimensions removes them.

I started tinkering with datapacks recently and wanted to add a new dimension. I couldn't ever access the dimension in 1.16-pre3, but it worked when I tried loading it on the previous version, 1.16-pre2. Upon reloading the world in 1.16-pre3, I loaded in the overworld and my custom dimension had been removed.

I assume this happens with all datapacks that add custom dimensions, but I'll link mine just in case.