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,

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
 ===============
 

changelog.d/9723.doc

@@ -0,0 +1 @@
+Add a document describing the deprecation policy for platform dependencies.

docs/deprecation_policy.md

@@ -0,0 +1,33 @@
+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.