In terms of relevant Phabricator tickets, in addition to the main ticket about IE11 support, this ticket discussed the changes in ResourceLoader which will allow specific modules to start declaring themselves as ES6-only.
On a practical level, that is the feature development teams will begin using on a case-by-case basis to drop IE11 JS support where it is deemed appropriate (starting with brand-new features). Major existing UI components like Visual Editor will probably not go down this path for quite some time.
Additionally, not shipping the JS version of a new feature to IE11 users may not always equate with not shipping a feature to those users at all – in many cases it will likely make sense to provide a no-JS fallback that would support users of IE11 (as well as users with JS disabled for whatever reason). The ability to designate certain RL modules as ES6-only is well suited to this kind of progressive-enhancement approach.