I think it would be very beneficial to create some sort of policy document, because it’s a necessary “clock” to know what kind of features in C are available for general use (e.g. aligned allocation, C11 atomics, etc.). Xref also a previous time this came up: Moving packaging and installers to macOS 10.13 as a minimum
Establishing a somewhat predictable cadence will be useful not only today, but long into the future. C’s evolution has not stopped, and being able to (somewhere down the line) use features from the very bountiful C23 etc. is going to be worthwhile.
Additionally, Apple pushes users hard to upgrade, so the usage numbers drop off sharply. Almost a year ago, 99.95%(!) of pillow’s macOS downloads were on >=10.13. Obviously a single library is a small sample, but the picture is pretty clear IMO. At some point the benefits of contortions to keep the very long tail of support going are really questionable IMO.
For example Chrome (which has a very wide user base) dropped support for <10.13 last year, dropped <10.15 in the meantime, and will drop support for <11.0 with Chrome 129 in ~October. Chrome support for macOS is also how google defines its overall lower bound as expressed in it’s “foundational support matrix” (which forms the baseline for other google projects like abseil, protobuf, etc.). Chrome was also the last holdout for LLVM’s libcxx 17+ to move its baseline to 10.13.
These projects might not be directly relevant to CPython, but they show up in a lot of places in the ecosystem. The combined effect of such key projects moving on was a lot of work in conda-forge that was necessary to support raising our baseline to 10.13 (just to keep compiling up-to-date versions), which was recently completed. It’s been almost 8 years since we had bumped our baseline to 10.9 – if we applied the same standards today as then (last non-EOL version), we’d need to jump to 12.0 today. So from my POV, even 10.13 is extremely conservative today, and already small quality-of-DX improvements would make that bump worth considering.