True when Matrix Authentication Service delegation is active, in which case
the Synapse-mirrored admin routes MAS owns (user admin, login-as,
reset_password, registration tokens) are left unregistered and answer 404,
mirroring Synapseβs de-registration of those servlets.
Assert the caller is a server administrator. Generic Synapse admin
endpoints use this plain check, not the MSC4323 anti-enumeration
authorize() guard whose self-target and admin-target ordering does not
fit them.