TL;DR there was a backdoor found in the XZ program. All major distros have been updated but it is recommended that you do a fresh install on systems that are exposed to the internet and that had the bad version of the program. Only upstream distros were affected.
The reason openssh links liblzma in the first place is to enable a systemd feature, so naturally “systemd bad, it’s proximity to a security issue is yet more proof that a pile of shell scripts in a trenchcoat is a superior init system” etc