After considering everyone's feedback, here are my proposed explanations for the tiers.
Tier 2
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- Must have a reliable buildbot.
- At least **two** core developers are signed up to support the platform.
- Changes which break any of these platforms are to be fixed or
reverted within 24 hours.
- Failures on these platforms block a release.
- Promotion to this tier requires consensus/SC approval.
All other platforms
-------------------
Support for a platform may be partial within the code base, such as
from active development around platform support or accidentally.
Code changes to platforms not listed in the above tiers may rejected
or removed from the code base without a deprecation process if they
cause a maintenance burden or obstruct general improvements.
Platforms not listed here may be supported by the wider Python
community in some way. If your desired platform is not listed above,
please perform a search online to see if someone is already providing
support in some form.
Once I have buy-in for the above I will explicitly ask folks to send review comments to that PR to add themselves to the platforms they are willing to support, and then drop any which lack two core developers (warnings to the Fedora/RH folks: most of those platforms are listed are using Fedora buildbots, so it might be up to you 😉). I did drop powerpc-linux-gnu from the list as that buildbot has not successfully built in quite some time (powerpcle seems fine).