Event confirmation emails should go to the email submitted, not primary email
A site visitor submits an event registration with email "firstname.lastname@example.org". This user is matched to an existing record, whether through unsupervised dedupe or they're logged in. Their primary email "email@example.com" receives the confirmation message.
This scenario is fairly rare because it's likely that the profile records the new email is the same location type as the old email, replacing it. However, I would argue that the expected behavior of any site visitor registering for an event is that the confirmation reaches them at the email provided. This is especially true if the old email is no longer in use.