> Modules need to opt in to even allowing tree shaking by adding sideEffects: false in package.json - which is something most people don't know to do.

That flag has always been a non-standard mostly-just-Webpack-specific thing. It's still useful to include in package.json for now, because Webpack still has a huge footprint.

It shouldn't be an opt-in that anything written and published purely as ESM should need, it was a hack to paper over problems with CommonJS. One of the reasons to be excitedly dropping CommonJS support everywhere and be we are getting to be mostly on the other side of the long and ugly transition and getting to a much more ESM-native JS world.