I've implemented OAuth flow glue logic a bunch in my career and I agree that the logic doesn't seem to follow from what the documentation says. What's not clear to me though while reading this is, if I delete/lose a domain, and someone re-makes the domain and account with the same email address, does the sub value then change? The docs would seem to suggest yes, but then I am not clear what google actually ended up "fixing" in its bug bounty. Was that maybe the bug, and this is just not clearly explained in this post?