The AD&D book (published in 1979) is that last reference I can find. It states:
NON-PLAYER CHARACTERS
Non-Player Characters: You should, of course, set the ability scores of those NPCs you will use as parts of the milieu, particularly those of high level and power. Scores for high level NPC's must be high - how else could these figures have risen so high? Determine the ability scores of other non-player characters as follows.
General Characters: Roll 3d6 for each ability score as usual, but use average scoring by consider any 1 as a3 and any 6 as a 4.
Special Characters, including Henchmen: Roll 3d6 as for general characters, but allow the full range (3-18) except in the abilities which are germane to his or her profession, i.e. strength for fighters, etc. For all such abilities either use on of the determination methods used for player characters or add +1 to each die of the 3 rolled which scores under a 6.
You also have several methods for generating ability scores, such as rolling 6 sets of 3d6 for each ability, and so on.
But what a book published nearly 40 years ago has to do with 5E is still beyond me, other than a little bit of historical trivia.