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.
Linked issues
Comments 3
Yes, this issue was the original issue and was opened before that one but the problem is the same. This issue explains the underlying issue and symptom while that issue represents a user story.
I reported this issue while testing the new experimental feature in hopes that it would be resolved before actually being released
Does MC-197860 happen to describe the issue that you are experiencing?