archlinux-archive to the rescue;
so you find the package in question from archive index here, and then find the relevant package version and copy the path to the exact .tar.zst file, and use following pacman command;
# pacman -U https://archive.archlinux.org/packages/path/packagename.pkg.tar.zst
in this case it would be;
sudo pacman -U https://archive.archlinux.org/packages/l/libebml/libebml-1.4.2-2-x86_64.pkg.tar.zst
or you can go for the easy way out, install downgrade, use command;
sudo downgrade <package>
which will list whats available from pacman cache or remotely, select the version you want the package to be downgraded into.