Are electromechanical oscillators still in use? Or totally obsolete?
I think you are describing an Alexanderson alternator, 1920s technology for generating significant power in the &lt; 100 kHz (Note, kHz not Hz!) range for long wave radio transmission. 

These things had many, many, poles and were run at scarily high shaft speeds but at the time vacuum tubes were only good to low kW power levels, where the alternators could make hundreds of kW.  

There is still an example in Sweden that was in use up to the 1990s for submarine communications, it is still fired up occasionally and can be heard on the air.

There are youtube videos of the thing being spun up. 

For shits and giggles I have actually built a mechanically driven direct conversion receiver, I used a stepper motor as the alternator driven by a servo motor, the stepper driving a diode ring mixer into an amplified speaker, mostly because I wanted to try to hear the Alexanderson set on a mechanical receiver (I cheated and used saturating ferrite cores as a frequency multiplier so I did not have to drive the thing at really stupid speeds).
CW tone was not wonderful but it was there and I could copy it.