Overseer Plush is an extremely rare item that only few have received for surviving a battle royale.
Yes, sometimes Beyond gets really bored during some of the testing instances and put players to fight for their lives.
The plush itself doesn't do anything, besides making a sound effect and light up the Overseer´s eyes when used. However, if you spectate someone in the list below (except contributors), they'll have a short string under their name; "overseer loves you :)".
Ruins O' Gems