Sure, things usually put env in /usr/bin, but there’s no guarantee for that.
There is even less guarantee for it to be anywhere else.
Hardcoding /usr/bin/env is probably your best bet
Because it is the convention.
That’s why #!env is probably your best bet
It definitely isn’t. That might work in your user space instance of bash in the desktop, but will likely fail in a script invoked during boot, and is guaranteed to fail on several non-gnu/non-linux systems.
#!/usr/bin/envis the agreed convention and there is no probably or but about that. If that does not work on a system it is a bug (looking at you BusyBox containers 🤨).