As you know, traditionally with older Solaris versions you had to leave /sbin/sh as the default root shell. In Solaris 8 and 9, you’re supposed to do this because all the libraries for dynamic linking are in /usr/lib, which could well be on a separate /usr filesystem. This filesystem may not be accessible during the maintenance boot, and therefore it is regarded as a very bad practice to change the root shell.
To make sure the superuser is always going to be able to log in, you have a /sbin/sh assigned to root, and it’s a statically built binary, so it doesn’t need any of the external libraries:
solaris$ file /sbin/sh /sbin/sh: ELF 32-bit MSB executable SPARC Version 1, statically linked, stripped solaris$ ldd /sbin/sh ldd: /sbin/sh: file is not a dynamic executable or shared object