version.json, with the vanilla Fabric profile. This is intended for future SKCraft/Solder support.
The Maven metadata XML files can be used to query the latest and available versions of Fabric components.
A HTTP JSON API is provided to allow lookup of Fabric versions for a specific Minecraft version, among other things. See the documentation on the Github, the API service is hosted at https://meta.fabricmc.net/ .
If you find something missing from the meta service open an issue on Github for it to be evaluated.