The IRS is determined to make sure that they juice every last drop out of you to make sure that you feel the hurt, because Americans denouncing their citizenship are one of two kinds typically, either A) Political migrants who are renouncing to make a statement about some great failing of the US, usually something the feds have dug their heals in over like Vietnam, or B) Financial migrants who are trying to escape tax increases.
Either way, the feds have a vested interest in deterring that behavior as much as possible and so the US is one of the few places that gets away with enforcing an arm and a leg exit tax and IIRC still makes you pay another 10 years worth afterwards basically to punish you for trying to skip your duty to your fellow citizens to support the country and her infrastructure and services that everyone shares.
Elizabeth Warren had actually proposed tightening the noose even harder during the 2020 primaries because where Republicans loose their shit over the idea of women being able to outsmart abortion restrictions, Dems, and especially progressive Dems, loose theirs at the thought of some Elon Musk type being able to take the bag they made off American infrastructure and labor and run away with it to avoid paying the fair share back.
People usually call it brain drain but really it’s all down to tax dodging, between the high rate of immigration and envy of the world higher education quality (so long as you can afford it), America is not running out of smart people without a 20th century style purge of the “intellectual class” by authoritarian militias because smart people aren’t dumb enough to go along with their strong man fetish.
Tl;Dr, the IRS REALLY wants to make sure you know that renouncing for financial reasons is gonna be way more costly than what you’re gonna save by fleeing to a tax haven.