I was certified in 1976, back when "Martini's Law" was still used as an example of how narcosis effects divers. I vaguely remember 100' being what our instructor mentioned as a approximate depth to "watch for narcosis", but that it could begin to effect some divers at shallower depths.
I begin to be effected somewhere just past 60 - 70', am quite "happy" (as if I'd had a few strong drinks) at 150', and "dumb as a sea cucumber" at 200' on air. Meaning at 200' I can monitor my depth and time and remain somewhat aware of my surroundings, but not a whole lot else. God save me if a true emergency occured at that depth, my mind would work too slowly to solve any but the simplest problem.
So my "safe working depth" may be bit shallower than other divers, and maybe a little deeper than others. We are all different, so I'd be hesitant to use 100' as a hard line in the sand.... I begin to monitor myself at shallower depths, and really try to be vigilant as I near 100'.
But I don't see narcosis as something to "fear".
Narcosis is worthy of very healthy respect and caution due to the potential impairment it can cause, but should not inspire fear, any more than the bends should cause fear.... we all know how to avoid both.
Best wishes.