Duktape supports ES5, with only partial support for anything ES6+ (and not much partial support, at that).

That said - having used Duktape in production, it's a very handy little library. Small, easy to drop into projects, great way to add support for custom user scripts or to run js code somewhere outside of a browser context.

Also - insanely easy to target still, despite the lack of ES6+, as long as you already have babel somewhere in your build pipeline.