A receiver catches a forward pass, lands deliberately just on one foot and then throws the football forward with so-called "throwing motion".
According to the rules:
A Pass is the movement of the ball caused by the runner who throws, shoves (shovel pass), or pushes (push pass) the ball.
The Runner is the offensive player who is in possession of a live ball (3-2-1), i.e., holding the ball or carrying it in any direction.
So it is required for a receiver to place the both feet inbounds to maintain possession, and it is also required to maintain possession to perform a forward pass. But since the receiver doesn't have possession (just one foot inbounds), it is not a forward pass and rather deflection.
So would it be an illegal forward pass?