I've been searching the internet for a couple of hours and couldn't find anything related. So far, I just found that the GitHub project is archived. Also, there are several posts about the rebranding made in 2018, but none mention that they will stop working on it. They are even keeping the old React VR Docs.
So, what happened to it?
It seems like this is the case. You can see all of the currently supported development technologies here. Two of these are Unity and Unreal. Just click on the computer icon to see the other technologies. I looked through all of the options and did not see React mentioned anywhere.