The new dimension and dimension_type directories in datapacks are named with singular names. All other directories (advancements, functions, loot_tables, predicates, recipes, structures, tags) have plural names.

In addition, they only function within the minecraft namespace, with the dimensions registered instead taking their namespace from a folder within the dimension directory. This means where any other data type would be registered as custom:my_dimension if placed in data/custom/dimension/my_dimension.json, instead one has to put it in data/minecraft/dimension/custom/my_dimension.json.

NOTE: Despite being marked as a feature request, this issue was half fixed in 1.16.2 Pre-Release 1. The directories are now in the right spot, but are still named with singular names.