This is a really interesting question, given the very long tail of old browsers out there. There's downloadable data about the browsers visiting gov't websites

This is a really interesting question, given the very long tail of old browsers out there. There's downloadable data about the browsers visiting gov't websites (all anonymized by Google Analytics)[1], but that doesn't solve the "when do you stop supporting" question, particularly for old versions of IE. [1]: https://analytics.usa.gov/ https://analytics.usa.gov/