I don't know, but it's been more reliable for me than /bin/bash. I think env is part of POSIX.

No. No new locations added in POSIX.1-2024.

https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1...

POSIX omits from standardisation what is not necessary for an application to work, so that a wide range of systems can be supported. As for Shebang, it can be rewritten in the installation script and is therefore considered an area of system administration.