This was kinda funny for me to test since it fails on macOS and I forgot I was testing on this OS at the time ;) I dug a little into this, because I'd like it to work on macOS and Linux.

macOS defaults to bsdtar and -I is a synonym for -T flag which means something different than GNU tar's -I flag. Both appear to support --zstd flags, though. It's possible --zstd will fail on older tar versions.

I do see that both bsdtar (on macOS) and GNU tar seem to agree on --use-compress-program but they both disagree on what this actually means. GNU tar assumes the program will be invoked with -d and bsdtar does not do this.

Both work with --use-compress-program 'zstd -d'

I'll create a new branch using your changes and the above additions :)

Thanks for improving fpm :)