Upstream notice
Arch updated their default compression to zstd . We adopted to the same standard. More and more packages will have the zst extension from now on. If you get for what ever reason an error with ZSTD not supported as archive format you can do this:
sudo pacman -Syy
sudo pacman -S pacman-static
sudo pacman-static -Syyu
This was the whole message in the announcements (since Arch moved to zstd).