I remember people theorising ways of making a thermonuclear device that didn't require a fission stage back in the 70s. If anything came of it, I doubt we'd be told. It went quiet in the 80s - because it was impractical or perhaps not?
The Teller-Ulam design is the current public peak of nuclear weapons design, and many nations concluded that it would do what it needed to do - make the other side gone. Why spend time on the creation of a fusion weapon when you don't need it?
A pure fusion weapon would require the miniaturization of something like a pulse-powered or laser confinement system, which with all publicly known technology isn't possible. If such a thing was developed trying it out would violate the Comprehensive Test Ban Treaty, which although not in force is mostly adhered to.
Some say that laser confinement research violates that, I happen not to agree. If anyone ever mastered fusion power, the knowledge of the pure fusion bomb would come with it. That's the same with pretty much any cheap, abundant source of power though as energy is a topic with an inherently military dimension.