How serious is the no checkmate with a pawn rule?

slumdog

 

#1

I’ve noticed several puzzles where it’s allowed in several where it’s not.
I’ve also won a game or two with a pawn checkmate.

Is it just a technical issue that the computer ignores it or is it only a relevant rule in serious games?

Kuroneko

 

#2

I'm not sure what happened in the games where you won, but the rule is that the checkmate can't be from a pawn drop. You can still checkmate with a pawn as long as it's not a drop.