@GregLudovici
If you’ve tested all the Users endpoints, including Get user email , Get user email bulk and Bulk get users, and none of them return the email address of a user who is deactivated, then you are probably going to have to use some sort of externally cached copy of all your users, their accountIDs and their email addresses as your source of truth.
PS. For the Get user email and Get user email bulk endpoints, you need to get special approval from Atlassian for your app to be able to do that.
There are a number of Jira Cloud feature requests related to activating / deactivating users via the REST API. Perhaps you could add your requirement to one of those requests, or create a new one.