Earlier quoted context omitted.
Why would you expect any institution to support all email addresses of their ex-employees ad infinitum? This would be a security nightmare for them. It is pretty normal for universities to have some sort of identity managmemt system that automatically provisions emails when you are employed there and deprovision them once you are gone.
Why not have a system where students and staff have actual email inboxes but alumni have their email forwarded? Most universities use a portal of some sort for easy access to personal information and preferences anyway, so it shouldn't be too difficult to limit access for alumni to only allow them to change a few personal details like name / address / phone number and the like, plus email forwarding settings. I think…
Additionally, making people who want to cold email work a little to acquire the current email address is actually a good thing, especially if they want to talk about something years old. I’ve generally had a lot more pleasant and engaging correspondence with people who worked out my email (say from a side project I develop pseudonymously) than ones who directly lifted my email from my professional profiles. So, expiring emails in papers generally isn’t a real problem anyway, and it’s basically never a hurdle if your target is still in academic circles. It only becomes a problem in this specific context of automated authentication (based on something not intended for that purpose).