I consider it would be a bit of overwork since it requires more stuff server side. I would just write the app requirements which are not only ES6 but might be HTML5 or CSS3 as well. The same way some apps require certain hardware or operating system we could require the use of a modern browser such as Chrome or Firefox (or even Edge, despite Microsoft is moving to Chromium as well). Right now it seems they had no problem switching to one of those.
Yes but I think is not worth the effort to work with server side stuff like babelify just in case some people use IE. You know the project better, do you consider this a priority or could we request that a modern browser is used? Either of the currently maintained or updated browsers could do it but IE is no longer maintained, only for security patches.