6 more years is a really long time on the web. You may want to base it instead on usage metrics — IE, once usage drops below what % are you willing to stop testing against a browser?
Win7 support is ending Jan 2020 also, at which point IE11 will only be supported on Win10 (and it's not default there). So that may be a good date to start considering ending IE11 support as well, especially with Chromium-Edge coming out later this year.