Can you attack with a creature the turn a Threaten effect wears off at end of turn?

A 'Threaten' effect (e.g., Act of Treason) gives you control of an opponent's creature until end of turn and grants it haste. The delayed triggered ability that returns the creature to its owner resolves at the beginning of the end step, which occurs after the combat phase. So by the time the creature reverts, attacking is no longer possible that turn.

Under CR 500.1, a turn's phases proceed in order: Beginning, Precombat Main, Combat, Postcombat Main, Ending. The end step is part of the Ending phase (CR 512), which comes after combat. You must declare attackers during the Combat phase's declare attackers step (CR 508), long before the end step trigger fires.

When the creature returns to its owner at the beginning of the end step (CR 509.2 handles delayed triggered abilities that return permanents), that player does not get a new combat phase — it is still the Threaten player's turn, and combat has already concluded.

Concrete example: You cast Act of Treason, stealing your opponent's 3/3. You attack with it during your combat phase, dealing 3 damage. Then, at the beginning of your end step, the 3/3 returns to your opponent. Your opponent cannot attack with it — it's your turn and there is no additional combat phase remaining.

In short, you (the Threaten player) absolutely can attack with the stolen creature during your combat phase that same turn thanks to haste (CR 702.10c), but once the end step arrives and the creature reverts, neither player gets to attack with it again until the owner's next turn.