hook_commerce_checkout_complete is never called
Is the hook still being used in Commerce 2.x?


No.


  If not, what should I look at instead to achieve a similar effect?


Commerce 8.x-2.x depends on the state_machine module which has events that are triggered when an order is placed (=checkout complete) as well as other state changes.

See Assigning (adding) an entity reference when an Order process is completed