It needs to remain exiled.
Imprint is an ability word (a word that denotes multiple abilities with basic similarities). It signifies the first of a pair of linked abilities. The first ability (the one with the imprint ability word) exiles a card. The second ability, which is linked to the first one, has some effect that is based on the exiled card.
Because the second ability refers to whatever card the first ability exiles, there needs to be a card in exile for it to function properly. If there's no card in exile, the second ability does as much as possible, but it may do nothing at all.
Semblance Anvil's second ability won't do anything if there's no card currently exiled by the first ability.
Prototype Portal's second ability can't even be activated if there's no card currently exiled by the first ability.