So, to be clear.... you will be releasing the new plugin alongside Unity 2018.1 official release. However, what you are releasing at that time will "launch in open beta"?
I could totally understand it taking 5 months after getting surprise news if this was a plugin asset being made by an indie/individual asset developer, working in their spare time and without any experience doing these things... but Allegorithmic has staff that do this stuff for a living, and they provide .sbsar support through a plugin asset with another game engine... it seems crazy to me that with more resources, and some experience in the area, it still takes 5 full months just to get a first beta out. And of course betas being betas, that could very well mean weeks or even months of things not being very stable.
I went from sympathizing with friends who were directly affected and being a little frustrated that I couldn't do any advance testing, to now being directly (and negatively) affected. The Unity-based virtual world I make things for has put their 2018.1 development on hold specifically because of the lack of .sbsar support, and if they're not able to get something stable very soon they are considering changing course to 2017.4 for their next major update (in the fall, as it takes them a while to get everything ready), and likely no 2018.1 support until 2019.
I do appreciate the information, but hopefully you can understand my frustration. The lack of openness and transparency hasn't helped, getting any info has been difficult and tough to swallow.