SS is not an "entitlement." It is a savings plan, a "trust fund" and a mechanism for dispensing money from the trust fund. FIT dollars are not involved.
An "entitlement" is a program created by government, by which tax dollars are granted to persons of a particular status or condition. Food stamps (or whatever they are called now), housing subsidies, public schools, welfare, Medicaid...these are all entitlements. Parenthetically, the USSC has ruled that once an entitlement is created by the State, it cannot be withheld from anyone due to their immigration status. This is bullshit, but it is the law.
The FEDERAL GOVERNMENT (that is, Congress) HAS NO CONSTITUTIONAL POWER TO CREATE OR FUND ANY ENTITLEMENT. And often, Congress just sends much of the money to the States, and the entitlements themselves are administered and partly-funded by the States.
It would be interesting if some citizen sued Congress to require that Congress cease and desist funding entitlements, for Constitutional reasons. The citizen(s) would win, however, there would be several procedural roadblocks.