https://en.wikipedia.org/wiki/Zsh says Apple switched to zsh as the default shell (with its MIT-variant license) in 2019.

GCC was replaced with LLVM in Xcode 4.2, and GDB was replaced with LLDB in Xcode 4.5 and GDB removed in Xcode 5.0. see https://en.wikipedia.org/wiki/Xcode