We are adding a new special domain that can be used to access any wiki while sharing cookies (T363695: Create a Wikimedia login domain that can be served by any wiki). For security reasons, there will be some differences in the configuration used while using this domain (T373737: Disable irrelevant extensions on SUL3 login domain). This means it's possible to break the authentication domain while making a change that's not in any obvious way related to it (most likely, by deploying a new extension that depends on an existing extension; if that extension is disabled on the authentication domain, the new extension needs to be disabled as well).
It's unreasonable to expect the people involved in a deployment to pay attention to this, so we should make sure the tooling checks for breakage, by adding some authentication URL (e.g. https://auth.wikimedia.org/enwiki/wiki/Special:Userlogin) to the canary URLs.
(Blocked on T377187: Set up auth.wikimedia.org for now, the authentication domain only exists in Beta.)