https://cdn.polyfill.io/v3/url-builder/ is pretty great for this, it'll detect the browser user agent and send down the right polyfill. Ie on modern browsers it'll send down an empty response.

Well, great sometimes. Not great if you don't trust a random 3rd party to host essential client-side code for you or if you don't feel like running yet another, maybe unsupported in the future, service for the next X years.

You can always self-host it: https://github.com/financial-times/polyfill-service

True. We have an "if IE then emit polyfill.io <script> tag" call in the backend, which limits the security surface area to IE and also prevents an otherwise unnecessary extra HTTP request (we only use it for Intl). We feel that's a reasonable middle ground.

If I'd use it for all polyfills (which we should maybe consider!), we'd likely self-host the service.

Well, you either need to format dates or you don't. The idea that you won't use Intl because the IE11 polyfill adds to much to your bundle size is a bit of a head scratcher.

The issue is whether it’s worth spending that bundle size on an obsolete browser. If not, it could be reasonable to do some naive formatting on IE11 and show a message mentioning that the browser is unsupported and things may render incorrectly.

Your IE11 polyfills don't have to be in the same bundle as everything else.