Apple yesterday unveiled macOS Catalina with support for iPad apps and many new features, but the presentation on stage lacked a big change for developers and power users. Starting with macOS Catalina, Macs now use zsh as the default login shell and interactive shell for the entire operating system. All newly created user accounts in macOS Catalina use zsh by default. Bash will still be available, but Apple signals that developers should migrate to macsh Mojave or earlier to zsh, so that Bash will no longer work under macOS.
Apple has not explained exactly why it makes that change, but Bash is not exactly a modern shell, as implemented in MacOS, and switching to a little less aging makes a lot more sense to the business. Apple does not use the GPLv2 licensed 3.2 version of bash, as newer versions are licensed under GPLv3. Apple has refrained from using GPLv3 packages in macOS, as the license is generally more restrictive for companies like Apple, which are signing their own code, and also includes explicit patent grants.
While some Mac developers have already gone over to Even more modern shells like Fish, zsh is better compatible with the Bourne shell (sh) and mostly with bash. With zsh, autocomplete is much more flexible. If you want to upgrade to zsh on macOS Mojave, you will find many helpful guides to getting the best terminal setup and getting used to zsh before bash is completely set.