Apple hasn’t explained exactly why it’s making this change, but bash isn’t exactly a modern shell as it’s implemented in macOS, and a switch to something less aging makes a lot more sense for the company. Apple is stuck using version 3.2 of bash that has been licensed under GPLv2, as newer versions are licensed under GPLv3. Apple has kept clear of using GPLv3 packages in MacOS as the license is generally more restrictive to companies like Apple that sign their own code and it includes explicit patent grants