Crib Swap can be cast from gy using Horde of Notions right? And does it exile the spell since it was cast from the gy or that's specifically for flashback mechanic? Thank you! :)
Yes, you can cast it with Horde of Notions’s ability.
Crib Swap won’t be exiled since nothing is telling it to go to exile or anywhere else. It’ll just go back to the graveyard.
Exiling an instant or sorcery that’s cast from the graveyard is specifically part of flashback. Any other effects that let you cast instants or sorceries in graveyards have to specifically say where the card goes when it resolves if it’s not to the graveyard.