This is a new bug since 1.16.2. there was a change in dimension loading experimental feature which created a bug in the ability to add/remove datapacks in existing worlds.

in 1.16.1, custom dimensions would be loaded but not loaded and saved into the level properties. This means they could be added and removed as desired without affecting loading of the save (the dimension data just wouldnt be loaded.

Since 1.16.2, dimensions included in datapacks are now included in the world generation settings and thus are saved into level.dat.  

When removing datapacks containing these dimensions the world will fail to load secondary dimensions.