I know someone who had his students do a survey of the resonant frequency of bike frames. They never found one that had a resonance less than 10Hz. They don't resonate much at all when fully assembled. Putting the wheels in eliminates the lower resonant frequencies. The motion is dominated by the static loading, not any kind of vibration phenomenon. I have never quite understood how the deflection of the bb would store energy that would be dissipated by moving the chain in a productive way. But then again, it probably can't take that much energy to get it into the deflected position in the first place. So you can't expect to get anything out of it.

I have seen a slow motion video of fork deflection when someone did a bunny hop. It was scary as all get out. I think the fork did a couple of heavily damped cycles and then stopped.

I think the main thing that changed as frames got stiffer was less problems with chains rubbing on front derailleurs.