When using custom noise_settings in a datapack, custom dimensions do not show up under "/execute" dimension list.  When not using custom noise_settings, custom dimensions do show up.  I have tried this with "dimensions" in the "minecraft" folder and with "dimensions" in my namespace folder.

generated_datapack7.zip --> custom noise_settings NOT USED in dimension JSON file = custom dimension shows up in the list

TheCaribbeanSea.zip --> custom noise_settings USED in dimension JSON file = custom dimension does not show up in the list

generated_datapack9.zip --> custom noise_settings USED in dimension JSON file AND dimension folder in the "minecraft" folder = custom dimension does not show up in the list