Per http://caniuse.com/css-gradients. IE9 does not support linear gradients. IE10 and IE11 implement the standard syntax. There was never a "-ms" prefixed version in any stable release of IE.
Previously removed from:
* (2013) jQuery ULS: https://github.com/wikimedia/jquery.uls/pull/114
* (2013) MobileFrontend: https://gerrit.wikimedia.org/r/#/c/72223/
* (2015) OOjs UI: https://gerrit.wikimedia.org/r/#/c/213523/
Remaining usage: ([search](https://github.com/search?q=%22-ms-linear-gradient%22+%40wikimedia&ref=searchresults&type=Code))
* MultimediaViewer: [mmv.mixins.less](https://github.com/wikimedia/mediawiki-extensions-MultimediaViewer/blob/a8838e4bae1f32b5240ef2cbd8e5a5edd2f12a83/resources/mmv/mmv.mixins.less#L34-L46)
* Vector: [jquery.ui.button.css](https://github.com/wikimedia/mediawiki-skins-Vector/blob/a3dc4e8735046adaeecc3e95d24ce9ab77d23e2a/skinStyles/jquery.ui/jquery.ui.button.css)