Comment by Thedarkb

It should always be optional as not everyone using tax prep software will actually be a citizen of the country where they're paying tax or registered with said country's social welfare system. I found navigating bureaucracy a nightmare when I first moved to the UK because so many systems were set up to require a National Insurance (Social Security) number which I wasn't able to get until after I started paying tax. Notably I was denied a bank account until I complained about it on Twitter.