It is sad that Microsoft thinks they can call Azure AD an enterprise level solution when they can't include basic, critical features like password expiration notifications in the product set. These answers that side-step the actual issue, question, and desire of customers display a true lack of customer care.
Just because some of your customers can use 3rd party solutions or devise work-arounds doesn't mean your product is providing the functionality that is needed, in fact it is not. Not everyone can utilize these outside solutions - especially not in GCC-High.
There has to be a solution that is not single focused. Why shouldn't we be able to simply enable a password reminder every x days for all cloud-based/AAD-based accounts? Why wouldn't the system enable an option to send a notification x days prior to a pw expiration on all AAD-based accounts regardless of AD Connect status?