If you want the exact plugin you’ll have to ask the MassiveCraft dev team, but it seems like they plan to stick to Spigot for the requirements of their server, at least for a while. You can probably find them on their Github or the MassiveCraft website.
Also check out [MCClans] (🛡 MCClans v1.1 [API 4.0]) by Kippers, it has some amazing functionality and I believe someone could make a plugin with its API to create the ability to claim land.