I brought this up on python-dev at https://mail.python.org/archives/list/python-dev@python.org/thread/ZPBSHENP3V7KHNPYWE6BEQD5ASES2NLV/ , and the feedback seemed supportive. As such, I am bringing a draft of what I'm thinking will go into PEP 11 with a bunch of `XXX` placeholders for people to help me fill in to see how this will look overall.

For any platform(s) you support, please reply with any relevant details that should be added to the relevant tables below. Once I have these details I will loop back with the proposed update to PEP 11 and make sure everyone is still on board with the proposal.

=====
Tiers
=====

Tier 1
======

- `Test suite failures <https://github.com/python/cpython/actions/workflows/build.yml?query=branch%3Amain+is%3Acompleted>`__ block releases.
- Changes which would break the ``main`` branch are not allowed to be merged;
any breakage may be reverted immediately.
- All core developers are responsible to keep these platforms working.
- Promotion of this tier requires consensus/SC approval.

=================== =====
Target Triple Notes
=================== =====
i686-windows-msvc
x86_64-windows-msvc
x86_64-apple-darwin macOS 11
x86_64-linux-gnu glibc 2.31 |ubuntu-20_01|_
=================== =====

.. [ubuntu-20_01] https://launchpad.net/ubuntu/+source/glibc/2.31-0ubuntu9.4

Tier 2
======

- Must have a stable buildbot.
- At least **two** core developers are signed up to support the platform.
- Changes which break any of these platforms are to be reverted within 24 hours.
- Failures of these platforms block a release.
- Promotion of this tier requires consensus/SC approval.

====================== ========================== ============================================== ========
Target Triple Notes Buildbot Contacts
====================== ========================== ============================================== ========
aarch64-apple-darwin XXX https://buildbot.python.org/all/#/builders/725 XXX
aarch64-linux-gnu glibc XXX [fedora-stable]_ https://buildbot.python.org/all/#/builders/125 XXX
glibc 2.28 [RHEL8]_ https://buildbot.python.org/all/#/builders/529 XXX
aarch64-windows-msvc XXX https://buildbot.python.org/all/#/builders/729 XXX
powerpc64-linux-gnu glibc XXX https://buildbot.python.org/all/#/builders/237 XXX
powerpcle-linux-gnu glibc XXX https://buildbot.python.org/all/#/builders/90 XXX
s309x-linux-gnu glibc XXX https://buildbot.python.org/all/#/builders/223 XXX
glibc 2.28 [RHEL8]_ https://buildbot.python.org/all/#/builders/509 XXX
glibc 2.17 [RHEL7]_ https://buildbot.python.org/all/#/builders/179 XXX
x86_64-linux-gnu glibc 2.17 [RHEL7]_ https://buildbot.python.org/all/#/builders/15 XXX
x86_64-unknown-freebsd XXX https://buildbot.python.org/all/#/builders/172 XXX
====================== ========================== ============================================== ========

.. [fedora-stable] XXX
.. [RHEL8] https://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux#RHEL_8
.. [RHEL7] https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/7.0_release_notes/sect-red_hat_enterprise_linux-7.0_release_notes-compiler_and_tools-glibc

Tier 3
======

- Must have a stable buildbot.
- Code may be checked into ``main`` for the platform.
- At least **one** core developer is signed up to support the platform.
- Test failures do **not** block releases.
- Promotion to this tier is self-service.

========================= ========================== ============================================== ========
Target Triple Notes Buildbot Contacts
========================= ========================== ============================================== ========
wasm32-unknown-emscripten XXX XXX Brett Cannon, Christian Heimes
wasm32-unknown-wasi XXX XXX Brett Cannon, Christian Heimes
========================= ========================== ============================================== ========

All other platforms
===================

- Only code which either supports a higher-tier platform or is a general improvement may be checked in.