The US has exit taxes, as do many countries. If you try to renounce your US citizenship, you can be taxed based on the value of unsold assets.
I hate that the US is one of the few countries in the world that has citizenship-based taxation. It’s awful and stupid. But, in theory, it does mean that an American couldn’t just avoid taxes by moving to another country.
But, if they tried to move to another country and renounce their US citizenship they’d have to pay an exit tax based on the entire value of their assets, including unrealized gains.