docs: document special exception for machine type deprecation & removal

This extends the deprecation policy to indicate that versioned machine
types will be marked deprecated after 3 years, and then subject to
removal after a further 3 years has passed.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240620165742.1711389-15-berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

docs/about/deprecated.rst

@@ -11,6 +11,19 @@ releases, the feature is liable to be removed. Deprecated features may also
 generate warnings on the console when QEMU starts up, or if activated via a
 monitor command, however, this is not a mandatory requirement.
 
+As a special exception to this general timeframe, rather than have an
+indefinite lifetime, versioned machine types are only intended to be
+supported for a period of 6 years, equivalent to 18 QEMU releases. All
+versioned machine types will be automatically marked deprecated after an
+initial 3 years (9 QEMU releases) has passed, and will then be deleted after
+a further 3 year period has passed. It is recommended that a deprecated
+machine type is only used for incoming migrations and restore of saved state,
+for pre-existing VM deployments. They should be scheduled for updating to a
+newer machine type during an appropriate service window. Newly deployed VMs
+should exclusively use a non-deprecated machine type, with use of the most
+recent version highly recommended. Non-versioned machine types follow the
+general feature deprecation policy.
+
 Prior to the 2.10.0 release there was no official policy on how
 long features would be deprecated prior to their removal, nor
 any documented list of which features were deprecated. Thus