What makes you think that GP’s comment is talking about older OSes?
My understanding of their comment is that they’re talking about getting older Python interpreters to run on more modern OSes, modern enough that they don’t carry the older Python as a system package anymore. Hence, deadsnakes.
This is not about system packages or OSes, this is about your application needing Python 3.9.7 specifically, and locking to that, and 3.9.7 not being available in repositories anymore (3.9.7 is just an example). So normally you would either need to self host 3.9.7 somewhere or compile it from source on every new machine (which is terrible for CI but fine for local dev, a one off in local dev to build a Python version is nothing but paying 4 minutes every time on ci to compile your version of Python from source is a very angry amount of time for a lot of people).
That's indeed extremely specific. I'd imagine this pain could be self-inflicted with C-based extensions that were compiled (and can't be recompiled) with structures that don't exist in other versions.
I don't want to imagine what other eldritch horrors await developers working on this application.
Usually it is not the patch version being important eg being specifically 3.9.7, but rather picking one supported version and pinning it. Yes we should all be on the latest Python version that is available in the mainline repos and distros as a binary but a lot of orgs I’ve seen don’t invest the time and effort to properly be upgrading to be on the latest version of deps. So you are usually having one specific supported Python version that everyone builds against which may or may not be new enough to be in the official repo.
I also imagine the scenario you mentioned causing a lock to a patch version. But I see this as just a normal steady state a lot of orgs drift to if not staying on top of version updates
My understanding of their comment is that they’re talking about getting older Python interpreters to run on more modern OSes, modern enough that they don’t carry the older Python as a system package anymore. Hence, deadsnakes.