alt Hacker News> `foolib >= 3` will very often continue to work with foolib 4.0,
Absolute nonsense. It's industry standard that major version are widely accepted as/reserved for breaking changes. This is why you never see >= in any sane requirements list, you see `foolib == 3.*`. For anything you want to work for a reasonable amount of time, you see == 3.4.*, because deprecations often still happen within major versions, breaking all code that used those functions.
Breaking changes don't break everyone. For many projects, only a small fraction of users are broken any given time. Firefox is on version 139 (similarly Chrome and other web browsers); how many times have you had to reinstall your plugins and extensions?
For that matter, have you seen any Python unit tests written before the Pytest 8 release that were broken by it? I think even ones that I wrote in the 6.x era would still run.
For that matter, the Python 3.x bytecode changes with every minor revision and things get removed from the standard library following a deprecation schedule, etc., and there's a tendency in the ecosystem to drop support for EOL Python versions, just to not have to think about it - but tons of (non-async) new code would likely work as far back as 3.6. It's not hard to avoid the := operator or the match statement (f-strings are definitely more endemic than that).
On the flip side, you can never really be sure what will break someone. Semver is an ideal, not reality (https://hynek.me/articles/semver-will-not-save-you).
And lots of projects are on calver anyway.