When you Tempban an IP, the attempt to join message shows [player], instead of their username.

[18:44:01] [User Authenticator #194/INFO]: Warning: [player] attempted to join the server but was denied due to Stop complaining kid
[18:44:01] [User Authenticator #194/INFO]: Disconnecting com.mojang.authlib.GameProfile@82dd3c8id=ab9c53d0-3af9-4794-b2a8-969380c4af6e,name=bustageorge,properties={textures=[com.mojang.authlib.properties.Property@343a36f3]},legacy=false: You have been banned from this server for Stop complaining kid
It expires in 11 days 23 hours 59 minutes 49 seconds

I've not been able to recreate this.

What is [User Authenticator] ?

User Authenticator is what comes up in the console as the prefix.

Yep, just a info from Bukkit/Spigot itself.
Used to highlight any attempts of users to join the server.