You might wonder why you are hunting for a file from 2014. The reason exists instead of a 1.20 or 1.21 version is stability. The 1.7.10 version of Minecraft has a Forge API that allows modders to easily add new "Entity Properties" (which slugs rely on heavily). Modern versions of Minecraft (1.13+) changed the rendering engine drastically, making it incredibly difficult to port the slug transformation animations.