Thanks to Lena for the tip it looks like Ubisoft Plus will only be available in the US for Stadia when it launches on Stadia. This will be a big disappointment to many people outside of the US. The wording of the FAQ reads
Please note that only US residents will be able to participate in this beta
It also states the beta starts on November 10th however I believe this means the Luna launch will be that day.
We have reached out for comment to confirm that the FAQ is correct but at this time we would say if you live outside the US do not expect to access Ubisoft Plus on Stadia or Luna.
If you would like to access it you can use GeForce Now today. With GeForce Now you can access the vast majority of the gaming library on Ubisoft Plus and Watch Dogs Legion also as Ray Tracing on the platform.