Bash is the default shell on mainstream Linux distributions, but it's important to keep in mind that Bash is not Linux. It's possible for a given Linux-based operating system to be missing Bash completely. As such, it's best to avoid having Bashisms in shell script files meant to be portable.
For example, Alpine Linux is a lightweight distro that's not meant for PC's or servers, but for far weaker computers such as embedded systems and containers (although nothing stops you from using it in your PC). Consequently, it doesn't use Bash, because Bash is heavy—or "bloated" as some would say—compared to Busybox's ash shell.
Many web servers use distros of of BSD (which is not Linux, by the way), and such distros don't use Bash as the default. For example, FreeBSD uses dash by default.