Doc: Add REST API v2 (api/v2) entry in Release Notes (#50264)

closes #50105

Author: kaxil

RELEASE_NOTES.rst

@@ -555,6 +555,19 @@ The Airflow CLI has been split into two distinct interfaces:
 
 This change improves security and modularity for deployments that use Airflow in a distributed or API-first context.
 
+REST API v2 replaces v1
+"""""""""""""""""""""""
+
+The legacy REST API v1, previously built with Connexion and Marshmallow, has been replaced by a modern FastAPI-based REST API v2.
+
+This new implementation improves performance, aligns more closely with web standards, and provides a consistent developer experience across the API and UI.
+
+Key changes include stricter validation (422 errors instead of 400), the removal of the ``execution_date`` parameter in favor of ``logical_date``, and more consistent query parameter handling.
+
+The v2 API is now the stable, fully supported interface for programmatic access to Airflow, and also powers the new UI - achieving full feature parity between the UI and API.
+
+For details, see the :doc:`Airflow REST API v2 </stable-rest-api-ref>` documentation.
+
 REST API: DAG Trigger Behavior Updated
 """"""""""""""""""""""""""""""""""""""
 

airflow-core/docs/installation/upgrading_to_airflow3.rst

@@ -143,6 +143,7 @@ These include:
 - **CeleryKubernetesExecutor and LocalKubernetesExecutor**: Replaced by `Multiple Executor Configuration <https://airflow.apache.org/docs/apache-airflow/stable/core-concepts/executor/index.html#using-multiple-executors-concurrently>`_
 - **SLAs**: Deprecated and removed; Will be replaced by forthcoming `Deadline Alerts <https://cwiki.apache.org/confluence/x/tglIEw>`_.
 - **Subdir**: Used as an argument on many CLI commands, ``--subdir`` or ``-S`` has been superseded by :doc:`DAG bundles </administration-and-deployment/dag-bundles>`.
+- **REST API** (``/api/v1``) replaced: Use the modern FastAPI-based stable ``/api/v2`` instead; see :doc:`Airflow API v2 </stable-rest-api-ref>` for details.
 - **Some Airflow context variables**: The following keys are no longer available in a :ref:`task instance's context <templates:variables>`. If not replaced, will cause dag errors:
   - ``tomorrow_ds``
   - ``tomorrow_ds_nodash``

docs/spelling_wordlist.txt

@@ -655,6 +655,7 @@ falsy
 faq
 Fargate
 fargate
+FastAPI
 fastapi
 fbee
 fc

reproducible_build.yaml

@@ -1,2 +1,2 @@
-release-notes-hash: 4a33893e65bbabc29d00816684bb9b78
-source-date-epoch: 1746029438
+release-notes-hash: 1b9fae6db6fdad0ea37c6473561bca29
+source-date-epoch: 1746549398