My main Stack Exchange account has been using an email address that puts me at risk of leaking sensitive private data about myself. Although I hid the email, I just very recently discovered that moderators of the sites can still access the email address. As I do not want anyone to see it, I changed the email address entirely. I would like to ask if moderators can still see my past email address? If yes, will the deletion of the account be sufficient (hopefully I can keep my account)?
I think your best bet would be to send a GDPR request, to ask SE to remove your previous email completely. You can do so via this form: https://stackoverflow.com/legal/gdpr/request
While the law that forces SE to adhere to this is a European one, I have no hints that they limit this to European citizens. AFAIK they will also effect request made by other nationals.
I'm not sure if it's still the case, but it used to be the case that moderators could also see previous emails.
There are various events that are logged in a user profile history, changing your mail address was one of those events. Moderators can view this history for every user on their site.
You also should make sure that the login method you use, whether it is an SE account or e.g. a Google account also uses an Email you're comfortable with moderators or SE employees seeing. This is not related to the email setting of your SE profile, but a separate thing. You can see your logins on your profile edit page under "my logins".