Our self-hosted instance is affected by this, as well. We would like to host the service that receives the webhook only on the internal docker network; there’s no need for it to be reachable from outside.
Is there a way to allow internal webhooks?
If I understand the issue correctly, internal webhooks are not allowed such that a discourse admin, who is not entitled as a server admin, may not spy on network internals. This is not an issue on our self-hosted instance, though; our discourse admins would know about our internal structure anyway.
I would suggest a setting in the app.yml config to allow internal webooks, or to provide a whitelist of internal domains/IPs. This way, the server admin (not the discourse admin) remains in control of who may use webhooks for the internal network.