All projects have a useful lifespan. Quail was born during a time when accessibility testing, as a discipline, was maturing. There were numerous teams building solutions in parallel -- Quail was just one of these. In the end, we were all orbiting around the same general approach. Technology may have been a distinguishing factor a couple years ago, but even these advantages have largely dissolved as solutions have evolved and improved over time.
Given the realities of the market and the limited time that the Quail team can devote to this project, we are initializing deprecation for this project. Folks are welcome to fork it or volunteer to maintain it, but realistically, there are better options out there.
If you are looking for one, we recommend aXe: the Accessibility Engine by Deque Labs.