Datapacks for Minecraft 1.21: 1,283 items in the Data packs section. Compatibility is checked against the files, not the description: only items with a build for exactly these conditions stay in the list.
That's the main difference from mods: a datapack doesn't show up in the mod list, and it can't add anything the game doesn't have at all — you can't make a new block with its own model this way. On the other hand, it doesn't clash with other packs, doesn't need updating when you switch loaders and doesn't stop you from playing with friends on a regular server. Installing a datapack is simple: put the archive in the datapacks folder inside the world folder, then reload the world or run the reload command. The card lists the game version, author and license, and all releases are gathered in one place.