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
Invalid. For version 21w08a, your noise settings were outdated (probably made for 1.16.5), that's why it didn't load. For example in
caribbeandea:sanddd, you're missing theaquifers_enabled,deepslate_enabled,noise_caves_enabled, andnoise.min_yfields. Custom worlds formats are constantly changing and still marked as experimental.