docs: add data retention support for preserving pinned posts

- Add Preserve pinned posts configuration setting in compliance settings
- Update data retention policy documentation to reflect pinned post preservation option
- Available from Mattermost server v10.10 onward

Closes #8070

Co-authored-by: Carrie Warner (Mattermost) <cwarnermm@users.noreply.github.com>

Author: claude[bot]

source/comply/data-retention-policy.rst

@@ -22,7 +22,8 @@ To set a global data retention policy:
 2. Select **Edit** from the menu located to the right of the **Global retention policy** table. 
 3. Specify a global retention policy for channel messages and direct messages by selecting a **Channel & direct message retention** option from the dropdown, then set how long to keep those messages in hours, days, or years. When a time is set, messages and file attachments older than the duration you set will be deleted. The minimum retention period is one hour.
 4. Select a **File retention** option from the dropdown. Set the number of hours, days, or years to keep files. When a time is set, uploaded files which are older than the duration you set will be deleted from your file storage system (either from your local disk or your Amazon S3 service as specified in **System Console > Environment > File Storage**). The minimum retention period is one hour. The global file policy deletes all files regardless of whether they're in a direct message, private, or public channel.
-5. Under the **Policy log** section, select **Edit** to specify the start time of the daily scheduled data retention job. Choose a time when fewer people are using your system. 
+5. *(Available from Mattermost server v10.10)* Optionally enable **Preserve pinned posts** to prevent pinned posts from being deleted by data retention policies, even if they exceed the configured retention period.
+6. Under the **Policy log** section, select **Edit** to specify the start time of the daily scheduled data retention job. Choose a time when fewer people are using your system. 
 
 Select **Save**. Messages and files older than the duration you set will be deleted at the specified server time, as applicable.
 
@@ -55,11 +56,15 @@ Frequently Asked Questions (FAQs)
 What happens when a message is deleted?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-The message is removed from the Mattermost user interface and deleted from the ``Posts`` table. The message is no longer searchable and cannot be retrieved in pinned posts or saved posts lists. 
+The message is removed from the Mattermost user interface and deleted from the ``Posts`` table. The message is no longer searchable and cannot be retrieved in saved posts lists. 
 
 Replies that did not exceed the message duration are still displayed in the user interface. However, further replies are no longer possible.
 
-If there was a file attached to the message, it will be removed from the user interface only.  
+If there was a file attached to the message, it will be removed from the user interface only.
+
+.. note::
+  
+  From Mattermost server v10.10, you can optionally preserve pinned posts when data retention policies delete messages by enabling the **Preserve pinned posts** setting in **System Console > Compliance > Data Retention Policies**. When enabled, pinned posts will not be deleted even if they exceed the configured retention period.  
 
 Why can I still see messages that were supposedly deleted?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

source/configure/compliance-configuration-settings.rst

@@ -79,6 +79,28 @@ By default, files are kept forever. If **Hours**, **Days**, or **Years** is chos
 
   From Mattermost v9.5, when a ``FileRetentionHours`` value is configured, the ``FileRetentionDays`` value must be 0 in the ``config.json`` file.  Conversely, when a ``FileRetentionDays`` value is configured, the ``FileRetentionHours`` value must be ``0``.
 
+.. config:setting:: preserve-pinned-posts
+  :displayname: Preserve pinned posts (Data Retention)
+  :systemconsole: Compliance > Data Retention Policies
+  :configjson: .DataRetentionSettings.PreservePinnedPosts
+  :environment: MM_DATARETENTIONSETTINGS_PRESERVEPINNEDPOSTS
+  :description: Controls whether pinned posts are preserved when data retention policies delete messages. Available from Mattermost server v10.10.
+
+Preserve pinned posts
+~~~~~~~~~~~~~~~~~~~~~
+
+*Available from Mattermost server v10.10*
+
+Controls whether pinned posts are preserved when data retention policies delete messages. When enabled, pinned posts will not be deleted by data retention policies, even if they exceed the configured retention period.
+
+**True**: Pinned posts are preserved and will not be deleted by data retention policies.
+
+**False**: **(Default)** Pinned posts are deleted according to the configured data retention policy.
+
++-----------------------------------------------------------------------------------------------------------------+
+| This feature's ``config.json`` setting is ``"PreservePinnedPosts": false`` with options ``true`` and ``false``. |
++-----------------------------------------------------------------------------------------------------------------+
+
 .. config:setting:: custom-retention-policy
   :displayname: Custom retention policy (Data Retention)
   :systemconsole: Compliance > Data Retention Policies