Letting your JS/TS compiler convert it into supported form. Not really a polyfill, but it allows to use new features in the source and still support older targets. This was done a lot when ES6 was new, I remember.
Polyfills are for runtime behavior that can't be replicated with a simple syntax transformation, such as adding new functions to built-in objects like string.prototype contains or the Symbol constructor and prototype or custom elements.
I haven't looked at the member properties bits but I suspect the pipeline syntax just needs the transform to be supported in build tools, rather than adding yet another polyfill.