Spigot 1.15 has been officially released. I have built this plugin against the latest version and tested on my local server. It should technically work with 1.15 since Spigot didn't change anything big in the API. But You may test it and tell me if there is a problem and I'll fix it ASAP.
There will be a Material update that adds the possibility to use newer Material names on old versions (Thanks to XMaterial) Stay tune I guess I'll do something else since doing this causes Required Item to always check the Data Value.