What financial implications are there, aside from "The IRS wants to make sure you're not leaving just before an anticipated windfall to avoid taxes?"
It's expensive to renounce US citizenship.
You mean if they've been living and working for years in the US, and taking advantage of US tax-advantaged retirement accounts?
Now we're talking about a different person than the parent commenter: "(loosely defined as people who were born in the US decades ago) who do not live or work in the US "