does it matter? MacOS is using some ancient ass 3.x bash from 2007. The location of bash is the least of your worries when it comes to portability. There's no guarantee any of the commands in your script (a) exist on the system or (b) work with the same options and flags that the script uses. I don't even know how many "netcat" commands I've seen in the wild. You could be running in some BusyBox or pared down Docker container. Blindly running a script that hasn't been deliberately crafted to work on your system is just asking for trouble.
It doesn't work at all on any BSD OS, which does not store bash in /bin - instead it is in /usr/local/bin
Specifying "env bash" makes it work on any UNIX, since the location of env is a constant, unlike bash.