Me neither, never. Buy if Arch is doing it, then I am sure it is worthwhile.

Zstandard is getting ubiquitous everywhere: inside of RPMs, as a way to compress data onto filesystems (both Btrfs and ZFS support it) and in several other applications. It decompresses much much faster than XZ while offering a comparable level of compression (XZ is still better, but at with an unreasonably worse computational cost).

The fact .tar.zst or .tzst isn't supercommon (yet) is for the same reason .tar.xz took years to become widely adopted: lots of OSes still ship `tar`s that don't support it out of the box, like Windows (which only supports gzip actually) and macOS (which ships an old version of bsdtar and no libzst). On a reasonably new Linux, is like opening every other kind of tar.

They're becoming increasingly common, especially if you ever pay attention to Linux packaging.

The majority that I've seen have been .tar.xz

Arch, arguably one of the most prolific user bases, uses zst.

Only time ive seen it was while installing zsh for git bash.