Not sure the 800lb gorilla you're referring to, but maybe UC Browser? That thing is so odd it's difficult to know what it supports, and it's not supported in nearly any popular CI service. I'm sure all kinds of things are broken on it, but you can still compile your app to ES5.
And ES6 _is_ a proper long lived standard. I'm not sure what would be more standard.
Compiling dependencies is not that heavyweight, especially when there is no configuration. Babel and TypeScript are fast. The results can be cached, it can be done on the fly for dev servers. Requirements shouldn't need to be checked - I'm not advocating for distributing code using decorators or class properties, nor modules (which without the HTML spec on module specifiers isn't a fully functional spec anyway) just plain ES6.
ES5 and ES6 classes cannot coexist on the same proto chain. Compiling to ES5 prevents things like mixins being usable against ES6 superclasses, so you force all dependents to be ES6. If you're going to force one way or the other, force in the future direction where there's still the option to compile the whole app to the older version of JS. ES5 classes can't extend subclassable built-ins like Array, Promise, Set, Map, HTMLElement, Error, etc. ES5 is now mostly slower than the equivalent ES6, it also doesn't minify as well as ES6.
