If I attack with Anivia into a battlefield with two units, and my opponent then plays Star Crossed to return one of the units as well as my Anivia, does the remaining unit still take damage from Anivia's attack trigger?

No, the remaining unit will not take damage.

Ruling

When Anivia's "When I attack" ability resolves, it checks the location "here" (the battlefield) to determine which units to damage. Because Star-Crossed returned Anivia to your hand before her ability could resolve, she is no longer at the battlefield. Since Riftbound has no "last known information" for locations, the ability fails to find its reference point ("here") and cannot deal damage.

Sequence of Events

1. Attack Declaration: You move Anivia to a battlefield occupied by two enemy units, initiating a showdown.
2. Attack Trigger: Anivia's "When I attack" ability triggers and is placed on the chain.
3. Reaction: Your opponent plays Star-Crossed as a reaction, targeting Anivia and one of the enemy units. Star-Crossed is placed on top of the chain.
4. Chain State (LIFO):
   • [Top] Star-Crossed
   • [Bottom] Anivia's "When I attack" trigger
5. Resolution:
   • Star-Crossed resolves first. Anivia and the targeted enemy unit are returned to their owners' hands.
   • Anivia's "When I attack" trigger attempts to resolve. It looks for "here" (the battlefield). Because Anivia has been removed from the board, the ability cannot find the location and fails to deal any damage.

This is consistent with RiftJudge FAQ #3820 and #5667, which specify that abilities referencing "here" fail if the source is removed from the battlefield before resolution.