I'm not sure it is discouraged so much as just not what people did in Python-land for a long time. It's obviously the right thing to do, it's totally doable, it's just inertia and habit that might mean it isn't done.
Pinning obviously the wrong thing, it only works if everyone does it and if everyone does it then making changes becomes very hard. The right thing is to have deterministic dependency resolution so that dependencies don't change under you.
But having that lock file will allow somebody to reconstruct your particular moment in time in the future. Its just that those lock files do not exist for 99.9% of Python projects in time.
I think this is the core to much misunderstandings and arguments around this question. Some people are writing code that only they will run, on a python they've installed, on hardware they control. Others are writing code that has to work on lots of different versions of python, on lots of different hardware, and when being run in all kinds of strange scenarios. These two groups have quite different needs and don't always understand each other and the problems they face.
It is only a matter of tooling. Locking ones dependencies remains the right thing to do, even for a lib.
At which point one might directly not pin, but that's "insecure" (https://scorecard.dev/)