I had a world in 1.16.5 with a custom dimension in it. In 1.16.5 both the End and the custom dimension were working perfectly well, but I upgraded the world to 1.17, then to 1.17.1. I didn't check if they were working in 1.17, but I checked in 1.17.1 and both the End and the custom dimension were treated as if non-existent. The End portal was not working, and neither the End nor the custom dimension did not show up in the auto-complete for the /execute in command and could not be accessed with it.

I tried disabling and removing the data pack with the custom dimension, which did not work. Reloading the world didn't work, and restarting Minecraft didn't work either. But when I created a new world, the End did work in the new world. What happened?

Update: It appears that after upgrading a world with custom dimensions to 1.17(.1), the custom dimensions, as well as the End, disappear from the level.dat file.