In Philemon 1:11, Paul says Onesimus was formerly useless but has now become what?

Answer: Useful

Onesimus was formerly useless to Philemon, but now he had become useful both to Philemon and to Paul (KJV: profitable).

“I appeal to you for my child Onesimus, whose father I became while I was in chains. Formerly he was useless to you, but now he has become useful both to you and to me.”— Philemon 1:10-11 (BSB)