An unknown provider

This card names a provider that does not exist. It renders as an ordinary card with no figures, and — the part that matters — it does not reach the network.

An allowlist, not a URL bar

The service adapters are a fixed set with fixed endpoints. A provider value the module does not recognise resolves to nothing rather than to a request, so a document's metadata can never talk the server into fetching an arbitrary address on its behalf. Anything else would make every page with an editor a way to make this server issue requests for you.