Spigot 1.13.2 latest
ItemJoin 5.0.3-b197

Please add Multiverse-Core to softdependencies in plugin.yml:

softdepend: [Vault, Multiverse, Multiverse-Inventories, xInventories, PlaceholderAPI, PerWorldPlugins, PerWorldInventory, AuthMe, MyWorlds, TokenEnchant, WorldGuard]

Multiverse-Core != Multiverse

Haha got me there!
Added and ready when the next commit is made to the snapshot branch.

EDIT: Also added the missing HeadDatabase and BetterNick, and put all of them in alphabetical order.

This feature request has been officially added in the release of ItemJoin v5.0.3.
You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=262715

Please read the changelog as the documentation has not been updated yet;
https://www.spigotmc.org/wiki/recent-changes