Class concept still exists and product specification is the same as before. However I understand your confusion, and here is why. Prior to Pega 8.4, Email wizard existed (see below screenshot).
This Email wizard let you select the Workpool as below.
Class path became the ID of Data-EmailAccount automatically.
However, this Email Wizard is gone from 8.4.
Now developer must enter ID. Since the form is a free text input as below, I don't think anyone would even try entering class path here unless you know that's how things work.
However, ID of Data-EmailAccount still works on Workpool basis. If you have a "MyCo-MyApp-Work-SupportRequest" (Workpool) then that will be applied. Instead if you have only "MyCo-MyApp-Work" (Workpool) then that will be applied. Or if you don't have either of them, "Default" account will be applied.