[ionicgiraffe]

I think in most countries in Europe the national id number is enough to link taxes to specific people (identity card numbers are to most effects public), even though for official international documents you need to provide the accepted Tax Identification Number (TIN) which you can find in https://www.oecd.org/tax/automatic-exchange/crs-implementati..., but more as a convenience than anything else.

That keeping secret your SSN is a thing is something that mostly affects the US.

[ghaff]

It depends what you mean by "secret." At some point, people decided that maybe we shouldn't be willy nilly broadcasting our SSNs, e.g. it used to be your driver's license number in many states. But given how widely it's used, most people (rightly) don't assume it's a deep dark secret.