Survival is a friendly game mode in FIFA and FUT where when a goal is scored a random player from the scoring side will be ejected from the match excluding the keeper.
You will get only four players removed from your squad. After the four players were sent off, no other players will be removed from the pitch.
How to Play Survival Mode
Survival friendly mode is available in FUT 21 > PLAY tab > FRIENDLIES (Couch Play / Play Online / Play A Friend).
In FIFA Kick-off mode, it is available under House Rules mode.