Possibly a really dumb question but I can't seem to find any documentation on the alleged v40 SDK features. Where is the best place to learn about this other than digging in the Unity integration package? I thought that as of v40, some sort of serialization of spatial anchors is possible for cross-device anchor sharing and I would really like to use this feature.
Yep, I'm very interested in network shared spatial anchors. In 2020, I experienced a group gaming experience in Tokyo, played PacMan VR, and noticed it was built using Quest headsets. So I think the functionality is already in there, but maybe it was locked away in some beta API at that time. The experience was remarkably stable and really quite fun!