Minecraft wiki entry for Development_versions, associated text: Tags can now be defined for any types in registry, rather than only blocks, items, fluids, entity types, game events and functions previously.
In datapacks, tags of new types are stored in their own folders (but not present in the vanilla datapack currently). The folders' names are based on their names in the registries.json, for example:
Tags for potions: /data/[namespace]/tags/potion
Tags for biomes: /data/[namespace]/tags/worldgen/biome
For tags of blocks, items, fluids, entity types, game events and functions, their locations in datapacks are currently not changed, even if they use plural names which is against the new name format.