Using your vernacular, a forward "lateral" is legal if it's done behind the LOS. What non-officials call a lateral is actually a pass. It's either forward or backward. The location of the forward pass is important because it's not legal beyond the LOS. I'm not sure about the NFL but in NCAA an illegal forward pass beyond the LOS that is not caught it still incomplete and causes the ball to be dead.
That's the reason I used "lateral" vs pass, even though a lateral isn't a thing in NFHS. I'm not referring to your typical "QB barely beyond the LoS/NZ when he intended to be behind it" because those are also treated as a typical incomplete pass in NCAA and NFL.
I'm referring to plays that are clearly meant you be backwards and occur after the ball has gone being the NZ or has changed team possession. For example, an end of the half or end of game play where a team attempts multiple backward passes/laterals/pitches, or a punt or kick return where a team attempts one or more backwards passes across the field.
ACTUALLY... I've just gone onto the NFL's online rule book and they also state that all forward passes, even illegal, are incomplete as soon as they hit the ground. They even give a situation example of a punt returner accidentally throwing the ball forward (intended to be backwards) and it hitting the ground. NCAA wording appears to be similar, but I'm 100% sure I've seen these type of plays and seen an official flag the illegal forward pass but nobody kills the play and they continue on as if it were a fumble. I even brought this situation up with some JuCo officials at my last game and they said that it would be a "running play" and a fumble under college rules.
So it looks as if I was incorrect about NFL and NCAA allowing it, but it seems like it still might be a worthwhile reminder to some that an illegal forward pass, even on these multiple backwards pass plays, ends the play if it is not caught (is incomplete).