This repo shows how esbuild leaves the import for unused node builtins in the output bundle.
To reproduce:
$ npm install
$ npm run build
Then inspect output.js.
Now switch the import in input.js to import from ./local-fs instead of fs. Run the build again and you should not see the import in the output because there is a sideEffects: false in package.json.
It'd be nice if esbuild used an implicit sideEffects: false for node builtins since they are purely library code and don't have any side effects.