We have words like lessee, rentee, invitee, addressee, etc. Why can't I use notificee as well?
It would be more likely to be 'notif(i)ee'.
There are thousands of such 'lexical gaps' in the language, If a need is felt, then a new word will be used and, in time, accepted - or rejected. The need for 'notif(i)ee' clearly has not been felt yet; we make do with 'person notified'.