Hi,
Currently there's no auditing for email notification changes in SCOM, you will have to go through every user that has SCOM administrator permission in your SCOM environment.
If you have any feedback related to this, make sure to submit your feedback over here:
https://systemcenterom.uservoice.com/forums/293064-general-operations-manager-feedback
(If the reply was helpful please don't forget to accept as answer, thank you)
Best regards,
Leon