You can download a supported zstd release through homebrew for every version of macOS that is supported by Apple (as those are the releases homebrew supports), and it compiles from source for older ones.

I don't even know why we're talking about bash here. But for the record, this is a unique circumstance that affects only software which made the transition from GPLv2 -> GPLv3. Changing your license can cause you to fork away from some users. News at 11.