It doesn't matter why. All that matters is that "current" is not a valid word. Old things exist in "current" systems. And current systems may also be old systems.
Ok. I think engineers are well capable of evaluating what systems they need to support and if zstd is a usable option for them. In many situations, the answer will be "yes."
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.
