Prepare release notes for BIND 9.21.9

diff --git a/doc/arm/notes.rst b/doc/arm/notes.rst
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--- a/doc/arm/notes.rst
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@@ -47,6 +47,7 @@ The list of known issues affecting the latest version in the 9.21 branch can be
 found at
 https://gitlab.isc.org/isc-projects/bind9/-/wikis/Known-Issues-in-BIND-9.21
 
+.. include:: ../notes/notes-9.21.9.rst
 .. include:: ../notes/notes-9.21.8.rst
 .. include:: ../notes/notes-9.21.7.rst
 .. include:: ../notes/notes-9.21.6.rst

diff --git a/doc/notes/notes-9.21.9.rst b/doc/notes/notes-9.21.9.rst
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@@ -0,0 +1,88 @@
+.. Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+..
+.. SPDX-License-Identifier: MPL-2.0
+..
+.. This Source Code Form is subject to the terms of the Mozilla Public
+.. License, v. 2.0.  If a copy of the MPL was not distributed with this
+.. file, you can obtain one at https://mozilla.org/MPL/2.0/.
+..
+.. See the COPYRIGHT file distributed with this work for additional
+.. information regarding copyright ownership.
+
+Notes for BIND 9.21.9
+---------------------
+
+New Features
+~~~~~~~~~~~~
+
+- Add support for zone templates.
+
+  To simplify the configuration of multiple similar zones, BIND now
+  supports a zone template mechanism. `template` blocks containing zone
+  options can be defined at the top level of the configuration file;
+  they can then be referenced in `zone` statements. A zone referencing a
+  template will use the options in the specified `template` block as
+  defaults. (Options locally defined in the `zone` statement override
+  the template.)
+
+  The filename for a zone can now be generated parametrically from a
+  format specified in the `file` option. The first occurrences of
+  `$name`, `$type` and `$view` in `file` are replaced with the zone
+  origin, the zone type (i.e., primary, secondary, etc), and the view
+  name, respectively.
+
+  Primary zones can now take an `initial-file` option, specifying the
+  path to a generic zone file that will be copied into the zone's `file`
+  path when the zone is first loaded, if the `file` does not already
+  exist.
+
+  For example, the following template can be used for primary zones: ```
+  template primary {                 type primary;                 file
+  "$name.db";                 initial-file "generic.db";         }; ```
+
+  With this template in place, a new primary zone could be added using a
+  single `rndc addzone` command:
+
+  ```         $ rndc addzone example.com '{ template primary; };' ```
+
+  The zone would be created using the filename `example.com.db`, which
+  would be copied into place from `generic.db`. :gl:`#2964`
+
+- Redesign the unreachable primaries cache.
+
+  Previously, the cache for the unreachable primary servers was limited
+  to 10 entries (LRU) and a fixed 10 minutes delay for each entry,
+  unless removed forcibly by a new entry. The cache is now redesigned to
+  remove the 10 entry limitation and to introduce delay values with
+  exponential backoff time - initially an unreachable primary server is
+  cached as being unreachable for 10 seconds, but each time the cache
+  entry is expired and the same server is added again during the
+  eligibility period of the next 120 seconds, the delay time is doubled
+  up until to the maximum of 640 seconds. :gl:`#3992`
+
+- Implement a new 'notify-defer' configuration option.
+
+  This new option sets a delay (in seconds) to wait before sending a set
+  of NOTIFY messages for a zone. Whenever a NOTIFY message is ready to
+  be sent, sending will be deferred for this duration. This option is
+  not to be confused with the :any:`notify-delay` option. The default is
+  0 seconds. :gl:`#5259`
+
+Bug Fixes
+~~~~~~~~~
+
+- Fix zone deletion issue.
+
+  A secondary zone could initiate a new zone transfer from the primary
+  server after it had been already deleted from the secondary server,
+  and before the internal garbage collection was activated to clean it
+  up completely. This has been fixed. :gl:`#5291`
+
+- Fix a zone refresh bug.
+
+  A secondary zone could fail to further refresh with new versions of
+  the zone from a primary server if named was reconfigured during the
+  SOA request step of an ongoing zone transfer. This has been fixed.
+  :gl:`#5307`
+
+