I’m likely missing context, not being a Debian user, but I would freak out when I read this, if I don’t know what’s actually going on:
Practically, this means that you can’t use pip to install packages outside a virtualenv, on a Debian system, any more.
This sounds like Debian is breaking me, and I will have no choice but to either use virtualenv, or switch system entirely. But in fact I still can install packages outside a virtualenv on a Debian system; I just can’t do that against the Python installation(s) Debian provides. I am not sure if (how) this subtle difference can be significant to certain people, but do wonder whether the message can be tweaked to be less absolute.