First, let's get rid of some of those generalizations. I own and write with some gold nibs that haven't got any bounce to them, and are nail-like to the point of annoying me. What is widely considered to be, at least technically, the best fountain pen ever made, the Parker 51, came with gold nibs that had a kind of life to them, but nothing like bounciness.
By contrast, the steel nib on my Monte Rosa 042 is much like other Montblanc nibs of the 1950s in being quite bouncy. I wouldn't want to say "semi-flex," but others do say that freely. The original and still the best flexible nibs used by commercial artists and calligraphers were and are disposable dip-pen nibs made of steel.
There are plenty of bouncy steel nibs in fountain pens. But not so many of those are in current production. As fashions changed, there were fewer bouncy nibs of whatever material. However, it is worth remembering that many fountain pens of what we speak of as the Golden Age were perfectly stiff. Then as now, different people had different preferences.
It can also be true that the same model can have nibs of differing stiffness over the product life of the pen. I have owned two examples of the Aurora 88 small fountain pen (in Italian the model name is Piccolo, which is to say, small). The pen has been manufactured since 1989. My first example had a painfully stiff gold nib. The second has a less inexorably stiff nib. And from my reading I gather that there are still other examples, perhaps from different product years than my pens, that had relatively yielding nibs. All of them gold, same pen model, perceptibly different feel.
Some generalizations hold, I imagine. Many do not.
Edited by Jerome Tarshis, 24 April 2019 - 07:41.