IIRC, xz was used by a systemd library, and that systemd library got added to sshd so it could tell systemd when it had started or something like that. SSH itself doesn't use xz.
xz is a compression library.
I'm aware? It's a compression library that is used by systemd, including in a systemd library that got added to sshd in some distros.
ssh out of the box also does not use libsystemd, except on systems which were patched to do so.
SSH does not support/use lzma/xz compression method for the SSH protocol.
The xz linkage was indirect through a systemd library that some systemd systems link into sshd.