Green card holders actually leaving the United States
Green card holders abroad whom the IRS no longer considers U.S. Persons
|c. 15,000 per year?
Fewer than four thousand Form 8833 filers
Around ten thousand ex-LPR deportees
As Patrick Martin, Phil Hodgen, and others have pointed out, if you previously immigrated to the U.S. as a legal permanent resident (LPR, “green card holder”), the IRS considers you a U.S. Person for tax and asset-reporting purposes even if you’ve left the country. Per 26 USC § 7701(b)(6), the only way this changes is if you either take a treaty-based return position that you are a resident of another country, or get a determination from USCIS or a court that your LPR status is gone. You usually do the former by filing IRS Form 8833. You usually get the latter by filing USCIS Form I-407 or by being deported.
Absent that, you remain a “U.S. Person” even if your green card expires or you’ve been out of the country so long that it’s clear you’ve abandoned your residence in the United States. In other words, you are a resident of the U.S. for tax purposes, but not for residence purposes. How many people are stuck in this tax limbo? The Department of Homeland Security estimates that more than 1.3 million LPRs moved out of the U.S. from 2005 to 2012. Generously speaking, perhaps six hundred thousand took action during that period to retain their green cards by applying for re-entry permits from USCIS (whether or not they knew about the tax consequences of keeping their LPR status). Another 115,000 actually filed Form I-407 to cancel their green cards; there appear to be a similar number of Form 8833 filers and deportees.
That leaves possibly as many as half a million emigrants who no longer have any right of residence in the U.S. but remain subject to that country’s worldwide “citizenship-based taxation” net.