Survival is a friendly match mode in FIFA 21 where when a goal is scored a random player from the scoring side will be ejected from the match excluding the goalie.
Only four players will be removed from your squad. No further players will be removed from the field after the four players who were sent off were sent off.
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 option.