Not sure if I would call it a threat.

Congressman Seth Moulton prposed to do this in May '23. Taiwan obviously wasn't happy about that proposal.

Then there's also a statement by former National Security Advisor Robert O’Brien from March '23 where he said that "I can’t imagine they’d be intact" after an invasion of China.

The 2nd one is a bit nebulous but doesn't talk about reunification but about annexation.