AFAICT, you keep your SS payments if you renounce your citizenship. This is an obligation, "full faith and credit" kinda stuff. I would definitely agree with things like ending federal funding of stuff and negotiating payments for any federal equipment/property that cannot be taken out efficiently, but not so sure how to end public obligations to individual people. Like I said, I know of retirees that took their pensions from Hong Kong to Taiwan (though, while fact-checking, I realize that this conversation is over a year old). So, whatever our intuitions about punishing traitors,
there appear to be some precedents already.
It would be an interesting mess if pensions were cut off. Old people in Texas would suddenly become a burden rather than a source of USD. OTOH, if they left Texas to move back to the USA, the Texans would lose a burden as well as get their assets at fire-sale prices. The far-right likes to trim out unproductive people.