Also, unfunded US govt retirement benefits come in two forms:

A) future pension and medicare costs, for people who are alive today, and

B) future retirement benefits for current and former govt employees

IMO (B) should be prefunded (or at least have an actuarial value on the govt balance sheet) because it's an actual liability.

But whether or not (A) should be treated the same way depends on whether the benefits are fixed as of now, or whether they can be reduced/eliminated if they take too large a part of the total budget at the time they're due to be paid.

Agreed, although (a) the fact that social security is the "third rail of politics" suggests that while it's not a contractual obligation like employee retirement benefits, it's effectively an unavoidable obligation; and (b) even if the future value of benefits to be paid out isn't recorded as a liability, the social security taxes paid should probably be treated as "deferred revenue" since that revenue is at least theoretically being gathered for the purpose of paying the future benefits in question.

Yes, taxes collected by govt effectively go into a single pool. The money is not ringfenced for particular purposes.

Both in the UK (where I'm from) and in the US (where I live now), there are separate taxes ('social security' in the US, 'national insurance' in the UK), but the AFAICT this is just a trick to obscure the true rate of payroll/income taxes.