I love all of the great address object features in NPSP, including the ability to mark addresses as "previous" and remove them as a default address that appears on account and contact records. I could definitely use some of the same features for email addresses --for example, we may want to keep an old email address associated with a contact record so that we can use it to search old emails, but "archive" it in Salesforce so it's not visible on the contact record and we're not tempted to use it.
I'm thinking of creating a custom email object with flows or processes to interface with the contact record (e.g., marking an email record "primary" updates the primary email on the contact record). One pro of such an object would be the ability to add a description field for users to write notes about how or when to use a particular address. We could also better track which emails have unsubscribed (sometimes donors just want to unsubscribe their work email, but still want to receive emails from us to their personal email). It would give us more flexibility for donors who have more email addresses than we currently have fields for on the contact record. However, I know I have to be careful with the background NPSP processes that determine the primary email address for a contact record. Are there other things I should watch out for? Any cons anyone can think of to doing this? Has anyone tried this?
EDF has done this for email and phone, although we aren't currently using the custom objects to manage consent (opt ins/outs). On the custom phone/email objects, we are tracking start/end dates, status, type, a note field, and whether or not it's the primary. Each contact can only have one primary phone/email and that maps to the standard Phone/Email fields. We have a lot of flows that are syncing the active relate phone/emails back to the NPSP fields on the contact. Our users can only modify the contact-level fields on contact creation; otherwise, they must manage phones/emails through the custom objects. We also have flows that will downgrade other phone/email records if a new one is marked primary. Our biggest challenge is keeping the related records and the phone/emails that get passed down to the contact in-sync after a merge (and being able to handle mass merges). Happy to chat about it if this is still relevant!
