Do you get attacker status when you attack an open battlefield?

No, you do not receive the Attacker designation when moving a unit to an open (unoccupied) battlefield.

According to RiftJudge FAQ #8593 and Rule 807.1.d, the "Attacker" designation is only gained during Combat. Moving a unit to an open battlefield initiates a non-combat Showdown, not Combat. Because there is no opposing unit present to create a combat encounter, the Attacker/Defender designations are not established.

Consequently, since the unit does not become an "Attacker," any passive abilities or keywords that rely on that designation—such as Assault—will not activate or apply their bonuses.