It’s a non trivial question.
replyEstimates of criminal activity, for example, are frequently counted as GDP in places like the UK. And even if you’re working in violation of visa rules, the IRS will still expect and enforce taxes.
These people don’t have an SSN. You can’t pay taxes without one
replyThere’s plenty of cases where illegal aliens are using the SSN of someone else, as reported by NYT, WashPost etc.
replyThey can and do pay taxes with ITIN instead of SSN, and some non-citizens are eligible for SSN anyway. Undocumented immigrants have paid billions in taxes annually. https://bipartisanpolicy.org/article/how-do-undocumented-imm...
replyThe larger point that they may be omitted on reports like this may still stand, but it’s not because every single one is unable to be tabulated in the count by definition.