It is not recommended for use as such since it was specifically designed for maintaining network information in its own format. It is a complete redesign and rewrite.
Backward compatibility is provided as a mechanism to ease transition.
It is a discontinued product and will no longer be available after Solaris 2.5. The Solaris 2.3 and 2.4 releases are exactly the same as in 2.2. There never has been and never will be. Typing 'nisls' gives you positive output. With Solaris 2.4 and higher, this step is no longer necessary.