Madden has made fumbling much more common in recent years, to the point where it is completely unrealistic. For that reason, sliding is an important feature in Madden 21. You are most likely going slide with your quarterback, as you can safely protect the ball by pressing RB/R2 with any other ball carrier. Both features will work with any ball carrier. You are going to want to avoid as many big hits as possible, one turnover could cost you the game.
How to Slide in Madden 21 (on Xbox)
To slide in Madden 21 on Xbox, hold down LT and RT, then tap the X button. You must be holding LT and RT when you tap X. It is important to lightly press X, as holding down X will result in a forward dive. Dives can lead to fumbles, even without contact.
How to Slide in Madden 21 (on PS4 and PS5)
To slide on a PlayStation device, hold L2 and R2, then tap the square button. You have to hold L2 and R2 when tapping the square button. Just like on Xbox, holding down the dive button (in this case square) will create a forward dive animation. Forward dives should really only be used in desperate situations are fumbles are caused by this animation at an unrealistic level.