As far as I can tell, /sbin appeared first in SunOS 4.0, where the hier(7) and filesystems(7) manual pages describe it as 'executable programs that are needed in the boot process before /usr is mounted'. They both claim that the SunOS 4.0 /sbin contained only hostname, ifconfig, init, mount, and sh.
According to these manual pages, SunOS 4.0 did not have a /usr/sbin. SunOS 3.5 doesn't seem to have had a /sbin.
(I'm so happy I finally found private copies of this SunOS 4.0 & 3.5 stuff.)