> It’s weird that major features are designed from start to finish without even having so much as a demo.

Not necessarily. In broad terms, Boost is considered a staging ground for features proposed to C++. Compiler projects also keep up with the standards.

In this case the concept guys continuously dropped the ball on this requirement though. One of the papers pointing the many flaws in the proposal calls out this shortcoming, where contracts have neither a workable reference implementation nor an adequate specification. In fact, they handwaved a lot over details with vague calls to "implementation-defined".