As announced on the Arch-dev mailing list, on Friday, Dec 27 2019, the package compression scheme has changed from xz (.pkg.tar.xz) to zstd (.pkg.tar.zst).
zstd and xz trade blows in their compression ratio. Recompressing all packages to zstd with their options yields a total ~0.8% increase in package size on all of their packages combined, but the decompression time for all packages saw a ~1300% speedup.
We already have hundreds of zstd-compressed packages in our repositories, and as packages get updated more will keep rolling in. No user-facing issues have been found as of yet, so things appear to be working.
As an end-user no manual intervention is required, assuming that you have read and followed the news post from late last year.