docs: Remove the appservice login warning page

The page said encrypted bridges cannot work with MAS and that a solution
was being worked on. That solution is MSC4190, stable since Matrix 1.17
and enabled for every application service in Synapse 1.141.0, so the
limitation no longer exists.

Replace the page with a readiness item on the migration page, which is
where the original warning was aimed, and redirect the old URL there.

book.toml

@@ -24,3 +24,6 @@ git-repository-url = "https://github.com/element-hq/matrix-authentication-servic
 
 # The path that the docs are hosted on
 site-url = "/matrix-authentication-service/"
+
+[output.html.redirect]
+"/as-login.html" = "setup/migration.html#encrypted-bridges-need-msc4190-support"

docs/SUMMARY.md

@@ -48,7 +48,3 @@
 - [Database](./development/database.md)
 - [Cleanup jobs](./development/cleanup-jobs.md)
 - [Internal GraphQL API](./development/graphql.md)
-
----
-
-[Application Services login](./as-login.md)

docs/as-login.md

@@ -1,7 +0,0 @@
-# About Application Services login
-
-Encrypted Application Services/Bridges currently leverage the `m.login.application_service` login type to create devices for users.
-This API is *not* available in the Matrix Authentication Service.
-
-We're working on a solution to support this use case, but in the meantime, this means **encrypted bridges will not work with the Matrix Authentication Service.**
-A workaround is to disable E2EE support in your bridge setup.

docs/setup/migration.md

@@ -30,6 +30,15 @@ If your Synapse homeserver currently uses a custom password provider module, ple
 It is worth noting that MAS currently only supports PostgreSQL as a database backend.
 The migration tool only supports reading from PostgreSQL for the Synapse database as well.
 
+#### Encrypted bridges need MSC4190 support
+
+Application services used to log in through the legacy `m.login.application_service` login type to create encryption-capable devices for their users.
+This login type is not available once authentication is delegated to MAS: encrypted bridges must instead create devices through the [device management endpoints](https://spec.matrix.org/v1.19/application-service-api/#device-management) introduced by [MSC4190](https://github.com/matrix-org/matrix-spec-proposals/pull/4190).
+
+This requires Synapse 1.141.0 or later, and a bridge which implements MSC4190.
+mautrix-based bridges do, with the `encryption.msc4190` option; see [their documentation](https://docs.mau.fi/bridges/general/end-to-bridge-encryption.html#use-with-next-gen-auth-oauth-mas-msc4190).
+For other bridges, check with their author.
+
 ### Install and configure MAS alongside your existing homeserver
 
 Follow the instructions in the [installation guide](installation.md) to install MAS alongside your existing homeserver.