On some platforms bash is not part of the default installation. (Aix is an example. It has ksh, awk and Perl.)

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No shell which is not bash is bash-compatible.

Not to my knowledge.
The instruction the hashbang gives is clear.
However, somebody (idiotic) could have symlinked another shell to /bin/bash...

It's not uncommon
but bash is not sh compatible in my experience, on other systems

nono, this is normal. I meant the other way round, e.g.:
That would be truly idiotic IMHO. Never seen that though.

The Python 2<==>Python 3 differences aren't earth-shattering but are annoying. Fixing "print" statements is pretty easy. The integer math changes were a bit sticker. There are likely other, more subtle, differences that I'm not yet aware of. I've drawn a line in the sand and made a decision to do everything as Python 3 as I'm still learning it -- it's soon going to be the only game in town anyway (https://www.theregister.co.uk/2019/0...python_2_2020/) -- and I can easily make the decision to avoid Python 2.x-isms. Unfortunately, I'm sure I'll have to bone up on those, though, as I encounter older Python scripts at work. (And have to modify those to remain supported.)

I haven't run into any problems with bash syntactical changes over the years. One can easily imagine the great gnashing of teeth that would result from such a move. (No... I don't think that backticks vs. "$( )" counts as a syntax change.)

Cheers...

I'm less bothered about what's available by default, (Bash for Aix is available at http://www.perzl.org/aix/index.php?n=Main.Bash), ondoho has answered my question...

Yeah, that's what I was asking about - to confirm that using Bash-specific features in a #!/bin/bash script is safe (barring deliberate idiocy/sabotage).

If the script doesn't run and needs to be checked & re-written, that's fine - if it could silently behave differently then that would be another matter.

Mostly, some features are only available in newer bash versions.

#!/bin/bash
is always bash, bash-3 or bash-4 or bash-5, but always bash.
If you write and test your scripts in bash-3 you are quite safe.

But the standard shell is quite dangerous.
You can make it safer if you disable the glob expansion in command arguments and for-loop lists:
An alternative to bash is zsh.
zsh protects just substituted variables from further expansions (word splitting and glob-matching).
So $var handling is much safer than in a standard shell.
You can explicitly enforce the standard handling with ${=var}

Unless it is not installed at all or elsewhere.