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I like this Hot Take, but there's no Standard way to serve up different versions based on UA right now, or to create multiple build targets. If we want people to do it, we have to build the Tools to make it The Default, not the Exception

Two bundles, one for “old” browsers and another for latest browsers is enough. Most people go towards dynamically generating bundles which doesn’t scale. User agent sniffing and explict whitelisting Chrome >VERSION and friends should be enough and should be fairly easy to setup.

Syntactic features, yes. Polyfills, maybe. It seems that in many cases you can ship a way simpler polyfill that only implements the subset of a spec that you use, without paying bytes for the edge cases you don't. Harder for a tool to do automatically, though.

You shouldn't ever be inaccurately polyfilling code. If you only rely on a subset then use a different abstraction. You cannot guarantee that people will only rely on the subset of the semantics you have. @ljharb has been pretty vocal about that with es-shims.

I mean I did report those TDZ bugs in Babel 3 years ago,

(they’re still unfixed)

Isn’t one of the main reasons for a transpilation that the spec hasn’t settled yet? Seems like a chicken and egg situation.

For experimental features, sure. However 90% of the syntax people use is standardised and will not change.