ive always heard it, at least as it was taught to actors, as a mid-atlantic accent, not because it came from the mid-atlantic states but it was somewhere between an northeastern accent and a british one. seems like a stretch to elide a posh prep school accent with an ethnic new york jewy one. maybe by the late 40s the accent just came to sound inauthentic and the new breed of actors and acting schools favored a more naturalistic approach. though that wouldnt explain why a flat affectless mid western accent became prominent necessarily.
some would suggest otherwise.
New Orleans has a uniquely NYery accent.
By the 70’s NYC accents we’re moving to the burbs. Think Long Island, Yonkers and NJ. Then there’s Da Bronx.
Looks like a lot of time has been put into the study of Jill Abramson’s accent. here