Add deprecation policy doc (#9723)

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

INSTALL.md

@@ -38,6 +38,7 @@ There are 3 steps to follow under **Installation Instructions**.
     - [URL previews](#url-previews)
     - [Troubleshooting Installation](#troubleshooting-installation)
 
+
 ## Choosing your server name
 
 It is important to choose the name for your server before you install Synapse,
@@ -542,7 +543,7 @@ Alternatively, you can do so from the command line. This can be done as follows:
     register_new_matrix_user -c homeserver.yaml http://localhost:8008
     ```
 
-This will prompt you to add details for the new user, and will then connect to 
+This will prompt you to add details for the new user, and will then connect to
 the running Synapse to create the new user. For example:
 ```
 New user localpart: erikj

README.rst

@@ -314,6 +314,15 @@ Testing with SyTest is recommended for verifying that changes related to the
 Client-Server API are functioning correctly. See the `installation instructions
 <https://github.com/matrix-org/sytest#installing>`_ for details.
 
+
+Platform dependencies
+=====================
+
+Synapse uses a number of platform dependencies such as Python and PostgreSQL,
+and aims to follow supported upstream versions. See the
+`<docs/deprecation_policy.md>`_ document for more details.
+
+
 Troubleshooting
 ===============
 
@@ -389,7 +398,7 @@ likely cause. The misbehavior can be worked around by setting
 People can't accept room invitations from me
 --------------------------------------------
 
-The typical failure mode here is that you send an invitation to someone 
+The typical failure mode here is that you send an invitation to someone
 to join a room or direct chat, but when they go to accept it, they get an
 error (typically along the lines of "Invalid signature"). They might see
 something like the following in their logs::

changelog.d/9723.doc

+Add a document describing the deprecation policy for platform dependencies.

docs/deprecation_policy.md

+Deprecation Policy for Platform Dependencies
+============================================
+
+Synapse has a number of platform dependencies, including Python and PostgreSQL.
+This document outlines the policy towards which versions we support, and when we
+drop support for versions in the future.
+
+
+Policy
+------
+
+Synapse follows the upstream support life cycles for Python and PostgreSQL,
+i.e. when a version reaches End of Life Synapse will withdraw support for that
+version in future releases.
+
+Details on the upstream support life cycles for Python and PostgreSQL are
+documented at https://endoflife.date/python and
+https://endoflife.date/postgresql.
+
+
+Context
+-------
+
+It is important for system admins to have a clear understanding of the platform
+requirements of Synapse and its deprecation policies so that they can
+effectively plan upgrading their infrastructure ahead of time. This is
+especially important in contexts where upgrading the infrastructure requires
+auditing and approval from a security team, or where otherwise upgrading is a
+long process.
+
+By following the upstream support life cycles Synapse can ensure that its
+dependencies continue to get security patches, while not requiring system admins
+to constantly update their platform dependencies to the latest versions.