Now I can't block opponent making undo if it doesn't reveal new info?
The only "undo" action of your opponent that doesn't need your approval is them auto-playing Treasures. Did you witness something else?
my opponent undo 3-4 times in the game, I have no way to disapprove him.
He undo too fast, cards are flashing back and forth, so I can't see what he really undo. And it isn't written in log after undo. Hate it. I remember his last undo is about treasure playing, he auto plays all treasure and have Anvil left in his hand, so he can't play anvil, and he undo to play anvil first, and discard a copper to gain something. But I'm afraid his other undo is during his action phase.
Is the treasure undo policy changed? Does it need approval some time ago?
The special undo behaviour of auto-played Treasures exists since the Renaissance release, so a few years. Also I'm not aware of any changes to this recently.