This project has moved to Planet Minecraft.
This data pack prevent Elytra from being equipped in dimensions they're not allowed in. It does this by unequipping Elytra in the restricted dimensions.
By default, Elytra are only allowed to be equipped in the End. You can configure which dimension the Elytra is allowed to be used in with the config file, which you can find at "data\dim_elytra\functions\config.mcfunction" within the zip file of the data pack.
More information about data packs and how to use them can be found on the official Minecraft Wiki.