Paper is custom server software and could be easily argued to be a mod loader if you consider plugins to be mods (although it’s probably a weak argument since there’s no mixin support built-in, although some large servers have added mixin support to their own Paper forks). However, it does not use Fabric under the hood (it’s based on Bukkit/CraftBukkit). By playercount, it is the largest (custom, standalone) MC server software in the world.