Creatures can't attack a player unless that player cast a spell or put a nontoken permanent onto the battlefield during their last turn.
Arboria’s effect cares about the actions taken by players, not their results. If a player cast a spell during their last turn but that spell was countered, that player may still be attacked. The same is true if a player put a nontoken permanent onto the battlefield during their last turn, even if it’s no longer on the battlefield by the time that player is attacked.
Arboria doesn’t stop creatures from attacking planeswalkers, regardless of what their controllers did or didn’t do during their last turns.
This effect modifies the announcement of an attack, so it only works if it is on the battlefield at that time.