Comment by mgerdts

The original idea of boot environments in Solaris came from Live Upgrade, which worked at least as far back as Solaris 8. Live upgrade was not part of Solaris, rather it was an addon that came from the services or enterprise support parts of Sun.

Solaris 11 made boot environments a mandatory part of the OS, which was an obvious choice with the transition from UFS to ZFS for the root fs. This came into Solaris development a bit before Solaris 11, so it was present in OpenSolaris and lives on in many forms of illumos.