docs(dev): remove deprecated createEventSource

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_28.rst

@@ -60,4 +60,17 @@ Deprecated APIs
 Removed APIs
 ^^^^^^^^^^^^
 
-* tbd
+* `\OC::$server->createEventSource()` has been removed, use `\OC::$server->get(IEventSource::class)` instead. 
+
+Fetching IEventSource source via dependency injection works only from Nextcloud 28. For older versions use \OC::$server->createEventSource() or `new OC_EventSource()`.
+
+If you want to support Nextcloud 27 and Nextcloud 28:
+
+.. code-block:: php
+
+		// @TODO: Remove method_exists when min-version="28"
+		if (method_exists(\OC::$server, 'createEventSource')) {
+			$this->eventSource = \OC::$server->createEventSource();
+		} else {
+			$this->eventSource = \OCP\Server::get(IEventSource::class);
+		}