I'm quoting another source for this answer:
"If you'd like to get rid of this, go to Rocket League in your steam Library, then right click on it and go to properties. At the bottom left you'll see 'Set launch options...' open that and then add in -NoKeyboardUI Press ok and play RL as you normally would. This will revert it backAccording to the normal chat box"Pysonix forums:
- Dirkened on the Psyonix forums
If you'd like to get rid of this, go to Rocket League in your steam Library, then right click on it and go to properties. At the bottom left you'll see 'Set launch options...' open that and then add in -NoKeyboardUI Press ok and play RL as you normally would. This will revert it back to the normal chat box
-Dirkened on the Psyonix forums