Most module developers will not be effected by this. It is dealing with internals.
Previously, the ParamConverterManager would throw a Symfony\Component\HttpKernel\Exception\NotFoundHttpException when route parameter upcasting failed.
It now instead throws Drupal\Core\ParamConverter\ParamNotConvertedException.
This only affects your code if you were directly using ParamConverterManager (service name paramconverter_manager) for the current request.
Additionally, instead of passing an empty string as the exception message, the following is passed:
throw new ParamNotConvertedException(sprintf('The "%s" parameter was not converted for the path "%s" (route name: "%s")', $name, $route->getPath(), $defaults[RouteObjectInterface::ROUTE_NAME]));
For example, if you try to match /node/2 and there is no node with an ID of 2, you will see this message:
The "node" parameter was not converted for the path "/node/{node}" (route name: "node.view")
That will only appear if you use the Routing API directly. In the normal context of a page request, NotFoundHttpException will be used instead, and it will result in a 404.
