I am making a slightly different point and have a bias to this perspective: www.legis.iowa.gov/docs/publications/…/19230.pdf

I am saying that an SSN can be part of a larger validation scheme, not the only key to the castle. Specifically for government sites, SSNs can be linked to IRS data to verify places of last residence. A person generally needs to verify multiple items that are referenced by the SSN before basic authentication can be established and set by the user. (This is part of the full Authentication, Authorization and Access Control triad.)

An SSN is just a broad level identifier. If you look at many laws around the release of SSNs, the redaction is usualyl in place to prevent the linking of different documents and other data points.

If I released my SSN in this chat, I could be fully doxxed in a matter of seconds. It’s mainly because there are many legal systems in place that use an SSN as a primary key, of sorts. (It’s a bit more than that, as SSNs can be duplicated in some circumstances.)

So to say, at a high level, an SSN is considered private is absolutely correct. However, it’s so easily referenced and obtainable it really isn’t fully private either.