Ugh sorry for weird autocorrect errors, I had to abandon for a mealtime haha... I was going to add that there are also mechanisms outside the language, for instance that you might just detect if you can do something by catching the exception if it doesn't work... If it's syntax like JS arrow functions this might require eval() or so. The API version is just to expect the failure codes that would have been present before you added the new feature, usually 404, and handle them elegantly. If it's a follow-up request that RFC-“must” be done for consistency, maybe your app needs a big shared event bucket where those can always go, so that when “POST /foo-service/foos/:fooId/bars/:barId/fix" fails, you can fall back to posting to the deferred request buffer something like

    {
      eventName: "foo-service.fix-bar-requested", 
      version: 1,
      params: {fooId: 5, barId: 7}
    }

Just catch the exception, record the problem for later resolution, use a very generic template for storage that doesn't have to change every month...

Think creatively and you can come up with lots of ways to build robust software which has new features. Remember the time reversal of adding a feature is removing it, so you have to fix this anyway if you want to be able to clean up your codebase and remove old API endpoints.