This mod disables PolyMc when it is installed on both the client and the server.
You have a modpack that you use on the server. You add PolyPuck into the modpack and put PolyMc on the server. PolyMc will be able to generate a resource pack and vanilla clients will be able to join the server using it. As you have PolyPuck in the modpack, you can tell players that they can optionally install the modpack. If they join using it, it will be just like any other modded server.
NOTE: don't install PolyMc on clients, as of version 3.1.0 it breaks horribly. NOTE: this mod currently blocks the resource pack packet for modded clients. This is planned to be split off into something more customizable.
CC0, probably not enough code to even be copyrighted. Feel free to use this in whatever.