If you're using xz version 5.6.0 or 5.6.1, please upgrade asap, especially if you're using a rolling-release distro like Arch or its derivatives. Arch has rolled out the patched version a few hours ago.
Gentoo just reverted back to the last tar signed by another author than the one seeming responsible for the backdoor. The person has been on the project for years, so one should keep up to date and possibly revert even further back than just from 5.6.*. Gentoo just reverted to 5.4.2.
I think that was a precaution. The malicious build script ran during the build, but the backdoor itself was most likely not included in the resuling package as it checked for specific packaging systems.
rolling release (because it was caught so quickly that it hasn't made its way into any cadence based distro yet)
using the upstream Makefile task to build a RPM or DEB (because the compromised build script directly checks for that and therefore doesn't trigger for a destdir build like Gentoo’s or Arch’s)
using the upstream provided tarball as opposed to the one GitHub provides, or a git clone (because only that contains the compromised Makefile, running autotools yourself is safe)
Points 1 and 2 mean that only rolling release RPM and DEB distros like Debian Sid and Fedora are candidates. I didn't check if they use the Makefile and the compromised tarballs.