They've been on a general GPL purge rampage, in each macOS release there's less and less GPL software (there was some blog post with numbers). As for bash, in

They've been on a general GPL purge rampage, in each macOS release there's less and less GPL software (there was some blog post with numbers). As for bash, in the latest release the default shell is zsh, though I think the old bash is still there, although clearly on the way out.