Support blocking via query parameters as well and document the change.

CHANGELOG.md

@@ -30,6 +30,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - **Breaking:** Notification Settings API option for hiding push notification
   contents has been renamed to `hide_notification_contents`
 - Mastodon API: Added `pleroma.metadata.post_formats` to /api/v1/instance
+- Mastodon API (legacy): Allow query parameters for `/api/v1/domain_blocks`, e.g. `/api/v1/domain_blocks?domain=badposters.zone`
 </details>
 
 <details>

lib/pleroma/web/api_spec/operations/domain_block_operation.ex

@@ -31,6 +31,7 @@ def index_operation do
     }
   end
 
+  # Supporting domain query parameter is deprecated in Mastodon API
   def create_operation do
     %Operation{
       tags: ["domain_blocks"],
@@ -45,11 +46,13 @@ def create_operation do
       """,
       operationId: "DomainBlockController.create",
       requestBody: domain_block_request(),
+      parameters: [Operation.parameter(:domain, :query, %Schema{type: :string}, "Domain name")],
       security: [%{"oAuth" => ["follow", "write:blocks"]}],
       responses: %{200 => empty_object_response()}
     }
   end
 
+  # Supporting domain query parameter is deprecated in Mastodon API
   def delete_operation do
     %Operation{
       tags: ["domain_blocks"],

lib/pleroma/web/mastodon_api/controllers/domain_block_controller.ex

@@ -32,6 +32,11 @@ def create(%{assigns: %{user: blocker}, body_params: %{domain: domain}} = conn,
     json(conn, %{})
   end
 
+  def create(%{assigns: %{user: blocker}} = conn, %{domain: domain}) do
+    User.block_domain(blocker, domain)
+    json(conn, %{})
+  end
+
   @doc "DELETE /api/v1/domain_blocks"
   def delete(%{assigns: %{user: blocker}, body_params: %{domain: domain}} = conn, _params) do
     User.unblock_domain(blocker, domain)

test/web/mastodon_api/controllers/domain_block_controller_test.exs

@@ -32,6 +32,20 @@ test "blocking / unblocking a domain" do
     refute User.blocks?(user, other_user)
   end
 
+  test "blocking a domain via query params" do
+    %{user: user, conn: conn} = oauth_access(["write:blocks"])
+    other_user = insert(:user, %{ap_id: "https://dogwhistle.zone/@pundit"})
+
+    ret_conn =
+      conn
+      |> put_req_header("content-type", "application/json")
+      |> post("/api/v1/domain_blocks?domain=dogwhistle.zone")
+
+    assert %{} == json_response_and_validate_schema(ret_conn, 200)
+    user = User.get_cached_by_ap_id(user.ap_id)
+    assert User.blocks?(user, other_user)
+  end
+
   test "unblocking a domain via query params" do
     %{user: user, conn: conn} = oauth_access(["write:blocks"])
     other_user = insert(:user, %{ap_id: "https://dogwhistle.zone/@pundit"})