It is an advertisement that was published in the main page of Arch Linux. So as not to take anyone by surprise, it is reported that:
Soon the files to be installed from Arch Linux (and from the RebornOS repositories) will use the zstd compression algorithm. In order to avoid problems when distributing files in this way, it is recommended to keep the system updated, especially pacman (version 5.2 must be at least), as well as having libarchive version 3.3.2-1 as a minimum.
Therefore, the extension of the packages to be installed will be .pkg.tar.zst (instead of .pkg.tar.xz).
If you keep the systems up to date, there won't be a problem.