We already decided to end support for IE8 and older (see conversation starting here). Shedding IE9-10 and requiring IE11+ would free us up even more, letting us shed some shivs and polyfills and start using flexbox.

IE9-10 were 0.7% of our traffic over the past year. (IE8 actually gets more than each one individually.) Microsoft no longer supports them. Adding them to our unsupported list doubles our unsupported IE traffic from 0.7% to 1.4%. I think this is all reasonable.