Announcing an uncastable spell
This is plain wrong way of handling that situation. It's an allowed bluff, like any bluff of cards in your hand or library (private zones).
“Yes, I play 4-of FoW”. I have none.
“Yes, I put those Torpor Orb from SB”. Didn't
“Yes, I sideboarded 13 cards against Miracles”. Nope, only 5. However, he can check my sideboard's SIZE, not contents :)
“Yes, I have the remaining piece of combo”, after which opponent concedes.
It's all allowed. In Regular REL, you may try to encourage players to be nice and truthful, but by no means this is a requirement nor they can be punished for that.