It is, however, available from Arch;
If it’s actually needed, it could be installed manually as mentioned in the same topic;
wget https://mirror.math.princeton.edu/pub/archlinux/extra/os/x86_64/libxml2-legacy-2.13.8-1-x86_64.pkg.tar.zst
sudo pacman -U libxml2-legacy-2.13.8-1-x86_64.pkg.tar.zst
That said, I don’t think it’s needed. Perhaps just wait till libxml2 (2.14.2-2) filters down from the Unstable branch.