As of today, NixOS (like most distros) has reverted to a version slightly prior to the release with the Debian-or-Redhat-specific sshd backdoor which was inserted into xz just two months ago. However, the saboteur had hundreds of commits prior to the insertion of that backdoor, and it is very likely that some of those contain subtle intentional vulnerabilities (aka “bugdoors”) which have not yet been discovered.
As (retired) Debian developer Joey Hess explains here, the safest course is probably to switch to something based on the last version (5.3.1) released prior to Jia Tan getting push access.
Unfortunately, as explained in this debian issue, that is not entirely trivial because dependents of many recent pre-backdoor potentially-sabotaged versions require symbol(s) which are not present in older versions and also because those older versions contain at least two known vulnerabilities which were fixed during the multi-year period where the saboteur was contributing.
After reading https://www.nongnu.org/lzip/xz_inadequate.html (first published eight years ago…) I’m convinced that migrating the many projects which use XZ today (including DPKG, RPM, and Linux itself) to an entirely different compression format is probably the best long-term plan. (Though we’ll always still need tools to read XZ archives for historical purposes…)