A search tells me I have only used the term once, back in 2010.
I may have more frequently used the term "narrative control mechanic", which is a little more clear in what it's referring to.
And, if I saw "narrative mechanic" I'd probably think they are really referring to a mechanic that calls for some specific direct control of the narrative.
On the player side, the "Legwork" move in some PbtA games calls for the player to describe/make up the person they eventually got information from. This figure does not have to have previously existed in the narrative - "Yeah, I talked to Bob, who is a homeless guy who often hangs out in the alley outside my office building, to find out where the new drug supplier's operation might be."
In Fate, when offering a Compel, the GM takes control of the narrative to add a complication that didn't exist before, or push character action in a way consistent with the character's stated Aspects - "Okay, so you have the Trouble Aspect: 'Snakes. Why'd it have to be snakes?'. Here's a fate point. When you open the sarcophagus you notice that it is Full of Snakes. That's now an aspect on the scene you may have to deal with."
I'm citing examples where someone is adding elements to the narrative, but those aren't the only things narrative control mechanics might do.