According ES6 Proxy Polyfill for IE11 there are no pollyfils for proxies in JavaScript; how far would the new reactivity system in VueJS v2.6+ will support IE11?
V2.x will not use proxies,.
V2.x-next (a parallel "experimental" branch) will
and
V3+ will likely make the complete switchable
Details & Source here: https://github.com/vuejs/roadmap
UPDATE
V2.X
No major breaking changes; will be maintained in parallel to 2.x with feature parity
V3.X
Possibility to swap the observer / scheduler with different underlying implementations. E.g. ES5 observer for IE11 compatibility, alternative scheduler to leverage requestIdleCallback for time-slicing.