Psalm 89 is a Maskil of which man?

Answer: Ethan the Ezrahite

Psalm 89, "a Maskil of Ethan the Ezrahite," celebrates God's covenant with David. Psalm 88 is credited to Heman the Ezrahite.

“A Maskil of Ethan the Ezrahite. I will sing of the loving devotion of the LORD forever; with my mouth I will proclaim Your faithfulness to all generations.”— Psalm 89:1 (BSB)