/bin is the "standard" location for bash on a subset of Linux distributions and basically no other Unix...
So it's not really a standard.
/bin/sh is a much more common convention but once again, not a standard.
There really isn't a truly portable shebang, but the same can be said about executables themselves. As part of the build or install step of whatever thing you're making, you should really be looking these up and changing them.
It's only really NixOS as far as I know that doesn't ever put bash in /bin/bash (as far as Linux distributions go). But, on the other hand, there are quite a few distros (or at least flavours of distros) which don't ship bash by default (alpine, minimal versions of most distros, and embedded-Linux focused stuff if you count it). I imagine the most common "installation" of Linux is userspace in a container (yeah I know there's no kernel there, but nobody who talks about "Linux" broadly speaking specifically cares about the kernel) and a good chunk of those will be minimal with no bash.
Bash has to be explicitly installed on OpenBSD, FreeBSD, NetBSD (I think, haven't used it in a while) and probably a bunch of others. And in all of those cases (that I know of) it doesn't end up in /bin/bash once installed.
The default bash shipped on macs is so abhorrently ancient that it would be strictly better if it didn't exist because it would reduce the number of people who think bash scripts I write are broken (they're not broken, they just inevitably depend on some bash 4+ feature). Moreover, hardcoding /bin/bash as your shebang in this case will prevent anyone from remediating this problem by installing a non-ancient bash because the old one in /bin/bash will still get used.
So it's not really a standard.
/bin/sh is a much more common convention but once again, not a standard.
There really isn't a truly portable shebang, but the same can be said about executables themselves. As part of the build or install step of whatever thing you're making, you should really be looking these up and changing them.
What's more, bash isn't a standard shell.