Once vested, RSUs are the same as regular stock purchased through the market. The company cannot claw them back, nor do they "expire".
> same as regular stock purchased through the market
You cannot purchase stock of a private company on the open market.
> The company cannot claw them back
The company cannot "claw back" a vested RSU but they can cancel it.
> nor do they "expire".
Yes, they absolutely do expire. Read your employment contract and equity grant agreement carefully.