Nyclad ammo was originally produced by Smith and Wesson in the 1970's. The Nyclad line was sold to Federal when S&W discontinued ammo production.
The 9mm Nyclad was a good choice for use in the older 9mm designs, pre the "wonder-nine" evalution/revealution. It and the old, small HP Remington could be used in Hi-Powers and early 39s/59s, etc. This was back before ammo evolved as it has and some of the firearms mentioned were designed around ball ammo.
The state of the art, stuff during this period was Super-Vel ammo.
By no means illegal to own unless you live in a juristiction that prohibits ownership of any HP ammo, it's a part of ammunition history.
I personally always liked the standard velocity .38 Spec 125 grain Nyclad HP for short barrled revlovers, due to fairly mild recoil, with a soft lead projectile. I keep my wife's 4" Diamondback, along with a half dozen or so speedloaders and speed strips loaded with it, because she shoots it well and does not complain about the recoil or muzzle blast.