> Why not do some compile time #ifdef SUPPORT_ES3?

Rather unfortunately, JS has no native precompiler. For the SQLite project we wrote our own preprocessor to deal with precisely that type of thing (not _specifically_ that thing, but filtering code based on, e.g., whether it's vanilla, ESM, or "bunlder-friendly" (which can't use dynamically-generated strings because of castrated tooling)).

Two problems: - people would need to know how to effectively include dependencies in a way that allows them to be tree shaken, that's a fragile setup - polyfills often have quirks and extra behaviours (eg. the extra functions on early promise libraries come to mind ) that they start relying on, making the switch to build-in not so easy

Also, how is this going to look over time with multiple ES versions?

It'll still install the dependencies, which is what this is about