Clarify Python dependency constraints (#18856)

Clarify Python dependency constraints

Spawning from
#18852 (comment)
as I don't actually know the the exact rule of thumb. It's unclear to me
what we care about exactly. Our [deprecation
policy](https://element-hq.github.io/synapse/latest/deprecation_policy.html)
mentions Debian oldstable support at-least for the version of SQLite.
But then we only refer to Debian stable for the Twisted dependency.

Author: MadLittleMods

changelog.d/18856.doc

@@ -0,0 +1 @@
+Clarify Python dependency constraints in our deprecation policy.

docs/deprecation_policy.md

@@ -1,13 +1,11 @@
-Deprecation Policy for Platform Dependencies
-============================================
+# Deprecation Policy
 
-Synapse has a number of platform dependencies, including Python, Rust, 
-PostgreSQL and SQLite. This document outlines the policy towards which versions 
-we support, and when we drop support for versions in the future.
+Synapse has a number of **platform dependencies** (Python, Rust, PostgreSQL, and SQLite)
+and **application dependencies** (Python and Rust packages). This document outlines the
+policy towards which versions we support, and when we drop support for versions in the
+future.
 
-
-Policy
-------
+## Platform Dependencies
 
 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
@@ -26,8 +24,8 @@ The oldest supported version of SQLite is the version
 [provided](https://packages.debian.org/bullseye/libsqlite3-0) by
 [Debian oldstable](https://wiki.debian.org/DebianOldStable).
 
-Context
--------
+
+### 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
@@ -50,4 +48,42 @@ the ecosystem.
 On a similar note, SQLite does not generally have a concept of "supported 
 release"; bugfixes are published for the latest minor release only. We chose to
 track Debian's oldstable as this is relatively conservative, predictably updated
-and is consistent with the `.deb` packages released by Matrix.org.
+and is consistent with the `.deb` packages released by Matrix.org.
+
+
+## Application dependencies
+
+For application-level Python dependencies, we often specify loose version constraints
+(ex. `>=X.Y.Z`) to be forwards compatible with any new versions. Upper bounds (`<A.B.C`)
+are only added when necessary to prevent known incompatibilities.
+
+When selecting a minimum version, while we are mindful of the impact on downstream
+package maintainers, our primary focus is on the maintainability and progress of Synapse
+itself.
+
+For developers, a Python dependency version can be considered a "no-brainer" upgrade once it is
+available in both the latest [Debian Stable](https://packages.debian.org/stable/) and
+[Ubuntu LTS](https://launchpad.net/ubuntu) repositories. No need to burden yourself with
+extra scrutiny or consideration at this point.
+
+We aggressively update Rust dependencies. Since these are statically linked and managed
+entirely by `cargo` during build, they *can* pose no ongoing maintenance burden on others.
+This allows us to freely upgrade to leverage the latest ecosystem advancements assuming
+they don't have their own system-level dependencies.
+
+
+### Context
+
+Because Python dependencies can easily be managed in a virtual environment, we are less
+concerned about the criteria for selecting minimum versions. The only thing of concern
+is making sure we're not making it unnecessarily difficult for downstream package
+maintainers. Generally, this just means avoiding the bleeding edge for a few months.
+
+The situation for Rust dependencies is fundamentally different. For packagers, the
+concerns around Python dependency versions do not apply. The `cargo` tool handles
+downloading and building all libraries to satisfy dependencies, and these libraries are
+statically linked into the final binary. This means that from a packager's perspective,
+the Rust dependency versions are an internal build detail, not a runtime dependency to
+be managed on the target system. Consequently, we have even greater flexibility to
+upgrade Rust dependencies as needed for the project. Some distros (e.g. Fedora) do
+package Rust libraries, but this appears to be the outlier rather than the norm.