zstd and xz trade blows in their compression ratio. Recompressing all packages to zstd with our options yields a total ~0.8% increase in package size on all of our packages combined, but the decompression time for all packages saw a ~1300% speedup.
We already have more than 545 zstd-compressed packages in our repositories, and as packages get updated more will keep rolling in. We have not found any user-facing issues as of yet, so things appear to be working.
As a packager, you will automatically start building .pkg.tar.zst packages if you are using the latest version of devtools (>= 20191227).
As an end-user no manual intervention is required, assuming that you have read and followed the news post from late last year.
If you nevertheless haven't updated libarchive since 2018, all hope is not lost! Binary builds of pacman-static are available from Eli Schwartz' personal repository (or direct link to binary), signed with their Trusted User keys, with which you can perform the update.