AFAIK of the modern OS, only on Solaris /bin/sh points to an actual bare-bones Bourne Shell, which is indeed the "earliest shell".
On most other modern systems /bin/sh is either a soft link or a hard link to a POSIX-compliant shell, which may have additional features.
On Solaris,
/usr/xpg4/bin/sh should be used instead.
If you stick to the features listed on this page:
POSIX Shell & Utilities your scripts should work across platforms...