Update Block Storage API v2 api-ref
Add a note that the v2 API was removed in Xena.
:tocdepth: 2
-=================================
-Block Storage API V2 (DEPRECATED)
-=================================
+==============================
+Block Storage API V2 (REMOVED)
+==============================
+
+.. note::
+ Version 2 of the Block Storage API was `deprecated in the Pike release
+ <https://docs.openstack.org/releasenotes/cinder/pike.html#deprecation-notes>`_
+ and was removed during the Xena development cycle. `This document is
+ maintained for historical purposes only.`
+
+ `Version 3
+ <https://docs.openstack.org/api-ref/block-storage/v3/>`_
+ of the Block Storage API was `introduced in the Mitaka release
+ <https://review.opendev.org/c/openstack/cinder/+/224910>`_. Version
+ 3.0, which is the default microversion at the ``/v3`` endpoint, was
+ designed to be identical to version 2. Thus, scripts using the Block
+ Storage API v2 should be adaptable to version 3 with minimal changes.
+
.. rest_expand_all::
