Previously, we discussed the new editor and browser support within WordPress core. Following up on those conversations, we are officially ending support for Internet Explorer versions 8, 9, and 10, starting with WordPress 4.8. Microsoft officially discontinued supporting these browsers in January 2016, and attempting to continue supporting them ourselves has gotten to the point where it’s holding back development. I realize that folks still running these browsers are probably stuck with them because of something out of their control, like being at a library or something. Depending on how you count it, those browsers combined are either around 3% or under 1% of total users, but either way they’ve fallen below the threshold where it’s helpful for WordPress to continue testing and developing against. (The numbers surprised me, as did how low IE market share overall has gone.)
Of course, wp-admin should still work in these older browsers, but with fewer capabilities, and we will no longer be testing new features and enhancements in these browsers. For example, the next versions of TinyMCE – currently targeted at WordPress 4.8 – will not support older IE browsers.
Source: https://managewp.org/articles/14933/wordpress-target-browser-coverage-ending-support-for-ie-8-9-and-10
Willie Chiu's Blog 