diff --git a/changelogs/internal/newsfragments/1940.clarification b/changelogs/internal/newsfragments/1940.clarification
new file mode 100644
index 00000000..15885433
--- /dev/null
+++ b/changelogs/internal/newsfragments/1940.clarification
@@ -0,0 +1 @@
+Add the `deprecated` field to properties of OpenAPI definitions and JSON Schemas.
diff --git a/data/api/client-server/joining.yaml b/data/api/client-server/joining.yaml
index aead420c..dda0d867 100644
--- a/data/api/client-server/joining.yaml
+++ b/data/api/client-server/joining.yaml
@@ -139,6 +139,7 @@ paths:
             type: string
         - in: query
           name: server_name
+          deprecated: true
           x-changedInMatrixVersion:
             "1.12": |-
               This parameter has been deprecated in favour of `via` and will be removed in
diff --git a/data/api/client-server/knocking.yaml b/data/api/client-server/knocking.yaml
index 39a928e8..085aa74f 100644
--- a/data/api/client-server/knocking.yaml
+++ b/data/api/client-server/knocking.yaml
@@ -50,6 +50,7 @@ paths:
             type: string
         - in: query
           name: server_name
+          deprecated: true
           x-changedInMatrixVersion:
             "1.12": |-
               This parameter has been deprecated in favour of `via` and will be removed in
diff --git a/data/api/client-server/login.yaml b/data/api/client-server/login.yaml
index b37d1742..385eb33b 100644
--- a/data/api/client-server/login.yaml
+++ b/data/api/client-server/login.yaml
@@ -119,15 +119,18 @@ paths:
                   $ref: definitions/user_identifier.yaml
                 user:
                   type: string
+                  deprecated: true
                   description: The fully qualified user ID or just local part of the user ID, to
                     log in.  Deprecated in favour of `identifier`.
                 medium:
                   type: string
+                  deprecated: true
                   description: When logging in using a third-party identifier, the medium of the
                     identifier. Must be 'email'.  Deprecated in favour of
                     `identifier`.
                 address:
                   type: string
+                  deprecated: true
                   description: Third-party identifier for the user.  Deprecated in favour of
                     `identifier`.
                 password:
@@ -194,6 +197,7 @@ paths:
                     x-addedInMatrixVersion: "1.3"
                   home_server:
                     type: string
+                    deprecated: true
                     description: |-
                       The server_name of the homeserver on which the account has
                       been registered.
diff --git a/data/api/client-server/registration.yaml b/data/api/client-server/registration.yaml
index 84aef5b1..1dd314b8 100644
--- a/data/api/client-server/registration.yaml
+++ b/data/api/client-server/registration.yaml
@@ -169,6 +169,7 @@ paths:
                     x-addedInMatrixVersion: "1.3"
                   home_server:
                     type: string
+                    deprecated: true
                     description: |-
                       The server_name of the homeserver on which the account has
                       been registered.
diff --git a/data/event-schemas/schema/m.room.encrypted.yaml b/data/event-schemas/schema/m.room.encrypted.yaml
index 95e05ff7..f608a6e2 100644
--- a/data/event-schemas/schema/m.room.encrypted.yaml
+++ b/data/event-schemas/schema/m.room.encrypted.yaml
@@ -41,6 +41,7 @@ properties:
           Olm event. For more details, see [Messaging Algorithms](/client-server-api/#messaging-algorithms).
       sender_key:
         type: string
+        deprecated: true
         x-changedInMatrixVersion:
           "1.3": |-
             Previously this field was required, however given it offers no additional
@@ -56,6 +57,7 @@ properties:
           for more information.
       device_id:
         type: string
+        deprecated: true
         x-changedInMatrixVersion:
           "1.3": |-
             Previously this field was required for Megolm messages, however given it