Servux is a server-side mod that provides extra support/features for some client-side mods when playing on a server.
Servux itself is never needed on the clients or in single player, it's only needed/useful on the dedicated server side in multiplayer.
In version 0.1.x it only has one thing, which is sending structure bounding boxes for MiniHUD so that it can render those also in multiplayer.
For compiled builds (= downloads), see https://www.curseforge.com/minecraft/mc-mods/servux