It's not a global setting, but rather something individual module authors can enable in their `package.json` – or, if you're working with a module that hasn't

It's not a global setting, but rather something individual module authors can enable in their `package.json` – or, if you're working with a module that hasn't specified it, you can tell webpack to treat that module differently via `rules`. So if you're a library author and you know none of your modules contain side-effects, you can set `sideEffects: false` and anyone using webpack with your library will benefit. :)