There would be little point in sending a re-activation notice to the
old address - you're assuming someone who changes their address would still have access to it and that could very well be not the case
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That's my case - every time I changed my address, I kept the access to the old one at least for a while. But yes, if someone used their job's email account and they are fired and lose any access to their email - then re-activating from the old address may pose a problem. I haven't thought of that - but I have never been fired. So far
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To change your address, you must have logged into the site using a name/password combination - that's from where the protection is derived.
So what is the point of account deactivation and the re-activation link being sent over email?