I vote to stay with backticks. We've maintained (or at least tried to
maintain) Bourne shell throughout the rest of nmh, and I think we
should keep it that way.
Officially, $(...) is part of POSIX; it's not a Bash-ism.
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html
We USED to use $(...) in the test suite. But that was dropped because
it turns out that /bin/sh on Solaris does not support that (at least, that
was my memory). I am neutral about whether or not we should continue
to support ancient shells.
--Ken