Thanks for posting this Martin. The loading of the Rhino tab in Rhino 8 vs. the non-loading in Rhino 7 is not the same as loading user objects. They’re really two separate mechanisms. That said, it does make sense that user objects should be saved in a directory with the version number it was created on. I’ll check with the team to see how that might be achieved. In the meantime, I’ve created a YouTrack item for this.
I’m pretty sure it’s not possible at the moment. A more consistent and version-robust way is needed to load all manner of additional content. Even the version-aware-plugin-loading is very much an afterthought right now.