I think you're missing the point, Bee. Categorizing attractions by letter is really a concept of the past (pre-1980's IIRC), when you had to have a certain ticket to get into the attraction. You would buy books of these and use them as you wanted. Think of it like local traveling carnivals. You pay nothing (or a very small fee) for admission, but have to buy tickets to ride. Some rides require more tickets than others.
So, even though we all might have our opinions about what type of letter a new attraction might get, Disney does not assign letters any more.
For fast pass (FP+) planning, the WDW site does include lists of attractions that use FP+ so that you can plan accordingly.
And just for the sake of argument
, if Country Bear Jamboree or Hall of Presidents could be (and were) be an E-ticket, 7DMT could easily have been an E-ticket. E-Ticket did not equate to big roller coaster.