Has a date for ending JavaScript support in IE 9 (dropping it from Grade A to C) been announced? I think it will be in MediaWiki 1.30, after Microsoft ends support for Windows Vista on April 11, 2017. IE 8 ended support a year ago, at around the same time Microsoft ended support for Windows 8 (not 8.1); as well as IE 8, IE 9 and IE 10 on Windows 7.
Topic on Project:Support desk
If it had been announced, it would be listed on Compatibility#Browser support matrix. https://phabricator.wikimedia.org/T128115 looks related when it comes to discussing it.
Now T128115 has been merged.