> Since JavaScript has an especially high burden for backward compatibility you can’t really remove old cruft

You can't remove features from JS runtimes (browsers or node), but you can certainly remove them from a new language version. "use strict" already did it once in JS. It can be replaced/upgraded with "use ecmascriptX.Y" for each language version.

And transpilers targeting JS and tools like packers and minifiers can enforce language versions without declaring them in each file.