It would be nearly impossible to say when the stable version would be released; the goalposts for the project are ever-changing as new technologies or ideas are adapted and integrated by the open-source community. Having said that, you can be sure that the moment its released an announcement will be placed here on the forums.
To give you an idea of where the team are up to, you can check out the Flarum Milestones along with Flarum Roadmap
In short, Flarum stable will be released when the team are ready for it to be, and when it has been extensively tested with modern technologies. However - in no way, shape, or form has this project been abandoned. The team are extremely active with updating and developing the core, and progress can be tracked on Github.
