It’s effective but tends to be a considerable amount of work to maintain, especially since the web is more dynamic these days: imagine what it would take to filter only authorized connections to a service hosted on AWS, for example, where anyone in the world can get IPs in the possible range and even put data on white-listed hostnames like S3. You’re basically building an allow list of host names, intermediating every update path, etc. and dealing with things which were designed with a more open model — e.g. do you disable things like OCSP or whitelist more third-party resources?

This also heavily encourages microservices since most non-trivial applications will have some reason to connect to fairly arbitrary resources. Hopefully that can be sandboxed well but relatively few apps were designed that way and that general class of missing things which weren’t supposed to work is notoriously easy for even experienced teams to miss.