Please reconsider including leaked password in Notification Emails. Consider letting users opt in.
Your users should treat this password as public information, as should you. If you are concerned about storing this information, then delete the leaked passwords once the notification emails have been sent.
The beneficial impact of all users knowing exactly which of their passwords have been leaked is likely much greater than the dangers of your copy of the passwords being leaked since these passwords are already in the open and should be treated as public information.
If you still feel against this, then please at least make it an opt in option. Let people opt in to agreeing to let you store and notify them of any password leaks for their email address.
It’s just too risky to handle this sort of data in a publicly facing service and not be able to store it as a secure cryptographic hash. Opting in would made a large amount of additional work to service a very small portion of the overall accounts in a breach.