Most popular PvP servers have anti-cheat plugins that detect if you are hitting a player you should not be able to see. Even if you see the floating sword, the server might flag you for "kill aura" if you react too perfectly.
Invisible players still emit "footstep particles" and "splash particles" when moving in water or rain. A good pack replaces these particle textures with large, bright stars or smoke clouds. minecraft see invisible players texture pack
Vanilla Minecraft does not hide the player's name tag or the in-world shadow (a dark circle under the player’s feet). Most popular PvP servers have anti-cheat plugins that
