> (1) How old must the Python version of those projects be, to not be supported any longer with any decent GNU/Linux distribution?

Debian-13 defaults to Python-3.13. Between Python-3.12 and Python-3.13 the support for `pkg_config` got dropped, so pip projects like

https://pypi.org/project/remt/

break. What I was not aware of: `venv`s need to be created with the version of python they are supposed to be run. So you need to have a downgraded Python executable first.