diff --git a/synapse/handlers/cas.py b/synapse/handlers/cas.py

index 0325f86e20..47ddabbe46 100644
--- a/synapse/handlers/cas.py
+++ b/synapse/handlers/cas.py
@@ -82,7 +82,6 @@ class CasHandler:
         # the SsoIdentityProvider protocol type.
         self.idp_icon = None
         self.idp_brand = None
-        self.unstable_idp_brand = None
 
         self._sso_handler = hs.get_sso_handler()
 
diff --git a/synapse/handlers/oidc.py b/synapse/handlers/oidc.py

index eca8f16040..648fcf76f8 100644
--- a/synapse/handlers/oidc.py
+++ b/synapse/handlers/oidc.py
@@ -338,9 +338,6 @@ class OidcProvider:
         # optional brand identifier for this auth provider
         self.idp_brand = provider.idp_brand
 
-        # Optional brand identifier for the unstable API (see MSC2858).
-        self.unstable_idp_brand = provider.unstable_idp_brand
-
         self._sso_handler = hs.get_sso_handler()
 
         self._sso_handler.register_identity_provider(self)
diff --git a/synapse/handlers/saml.py b/synapse/handlers/saml.py

index e6e71e9729..0066d570c5 100644
--- a/synapse/handlers/saml.py
+++ b/synapse/handlers/saml.py
@@ -80,7 +80,6 @@ class SamlHandler(BaseHandler):
         # the SsoIdentityProvider protocol type.
         self.idp_icon = None
         self.idp_brand = None
-        self.unstable_idp_brand = None
 
         # a map from saml session id to Saml2SessionData object
         self._outstanding_requests_dict: Dict[str, Saml2SessionData] = {}
diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py

index 0e6ebb574e..0fdc6dd9e7 100644
--- a/synapse/handlers/sso.py
+++ b/synapse/handlers/sso.py
@@ -104,11 +104,6 @@ class SsoIdentityProvider(Protocol):
         """Optional branding identifier"""
         return None
 
-    @property
-    def unstable_idp_brand(self) -> Optional[str]:
-        """Optional brand identifier for the unstable API (see MSC2858)."""
-        return None
-
     @abc.abstractmethod
     async def handle_redirect_request(
         self,