You have always needed on, practically speaking. Python isn't designed to have multiple versions of the same library in the same runtime environment. A virtual environment is just a separate place to put the packages you need for the current project, so that they're isolated from other packages, and thus you don't get version conflicts. This includes the system packages. If you want to play with, say, the latest version of Requests, and you try sudo installing that in a system environment, and it happens that the latest version of Requests breaks Apt (which is written in Python), you're in for a bad time.
The new warning is because even user-level installations can mess with system scripts, when those scripts are run without sudo. Also, Apt has no real way to know about or understand anything you do with Pip, so that interferes with Apt's actual package management.
>installing packages [with] sudo doesn't make them available to other users
If you use sudo to install packages for the system Python, then yes they absolutely are available to all users. But you don't see them in virtual environments by default (you can change this) because the default is to ignore the system installation's `site-packages` completely (including user-level installations).
> on ubuntu it seems pip has been replaced with 'python-*' debian packages
None of this is new, and it doesn't even remotely "replace" Pip. You're just facing a little more pressure to actually use the system package manager when installing packages for your system, since that can actually manage packages, and integrate them with the rest of your system (the non-Python parts). The Debian packages are specifically vetted and tested for this purpose and may include Canonical's own patches that you won't get from PyPI. On the other hand, PyPI provides vastly more different packages.
When you install in a virtual environment, you'll generally use Pip to do it (unless you use uv etc.). Because the environment is specifically created to be isolated from your system, so that Apt doesn't have to care.
