To convert my comment to a proper answer:
Since you are a (professor) emeritus, you should ask the current chair of your former department to reinstate your old email address: Having an institutional email address is a usual curtesy extended to all emeriti.
Personally, I never heard of the situation at a US university where an emeritus would not be allowed to keep a university email address. Providing emeriti with institutional email addresses is a common practice at my university and at two other places I randomly checked (Ohio State and Princeton).
For (some degree of) confirmation that it is a common practice in general, see the accepted answer here. (Even though the linked question was closed since "the answer to this question strongly depends on individual factors," no actual examples of denial of email addresses to emeriti were given.)
If a departmental Chair cannot figure out how to proceed with such a request (or declines the request), then escalate the request to Dean's office.