Adrestia’s second ability will trigger as long as it was crewed by an Assassin this turn, even if that creature has left the battlefield or stopped being an Assassin since.

When Adrestia attacks, if it hasn’t already been crewed by an Assassin this turn, its second ability won’t trigger at all. It’s not possible to crew it with an Assassin after it attacks in time to have the ability trigger.

Adrestia’s second ability will trigger only if it was crewed this turn by a creature that was an Assassin when it was tapped to pay the cost of Adrestia’s crew ability.