SS is based on income and high earners pay a lower rate with no guarantee of any return. It sure looks like a regressive income tax to me. By the way, where does this money go when paid in? It’s spent like any other income tax.
Like an issuer of an annuity the money stays with the issuer when the beneficiary dies unless he is willing to pay more to pass the remainder on.
High income earners pay the same rate up to a point but their returns are not in proportion to the payments. That is regressive. After the point of zero additional payment it might be considered progressive but not really since benefits aren’t increasing either. I guess it would be most accurate to say it is neither fish nor fowl.