It'd be some fluke of an accident. You'd need to be targeting not only debian:testing/unstable, but specifically debian:testing-20240311. And then - making sure not to apt upgrade at any point so you don't accidentally get any updates from the last 18 months - you'd need to also install openssh-server to avail of the backdoor, plus a service manager because running sshd in the foreground killswitches said backdoor. And then don't forget to expose the ssh port otherwise our effort is wasted.
The most realistic way to hit this would be to have built an image 18 months ago, on top of :testing or :unstable, and then not update or rebuild it at any time in those 18 months - in which case removing anything from the repo wouldn't help you. Or be purposely trying to recreate an affected environment for testing/research, in which case it's on you.
You're not wrong that we should keep our shields up - but "update sometime in the last 18 months" perhaps isn't such a revelation.
One thing does come to mind though - I do wonder if there's a way to strongarm apt's dependencies mechanism into having openssh-server conflict with affected libxz versions, so that if you did apt update && apt install openssh-server in an affected image, it'd bring a fixed libxz along for the ride. (and the images don't carry apt manifests, so apt update is required and you would have today's package list.) You could still pin an affected version, so there'd still be enough rope to allow you to recreate a research environment.