It's been only three weeks since AO last showed his face. Before that, he was gone for a few months after he updated to 1.12.0 and 1.12.1. Before that, he was gone for a few weeks or months as well.
The thing is, AmbientOcclusion has consistently returned. I trust that he is the kind of person who would tell us if he would permanently leave his modding days behind and until he says so, or goes MIA for at least 1,5 years, I will not call this mod dead.
At this point, I just really hope that AO makes XRay open source.
Then again, most of the time, bugs will never be fixed, instead, people will start saying "USE THIS OPEN SOURCE XRAY MOD!" and prioritize quantity over quality.