What happened to the Inductosyn?
No sure answers but I can postulate a few possibilities:

Optical encoders are in a class of technology completely different from resolvers and InductoSyn. That makes them easier to differentiate. If the low resolution of resolvers compared to optical encoders is widely known, it's going to cause more people to just pick the optical encoder rather than the InductoSyn which is more difficult to differentiate from the resolver.

The InductoSyn disc sounds like it would be more expensive to make than a quadrature optical encoder disc since it requires two discs while the optical encoder only requires one.

It could be the case that magnetic encoders, rather than optical encoders, supplanted Inductosyn . The description of the InductoSyn sounds a LOT like the magnetic encoders I use at work, down to the available pole counts and interpolation factors, except that instead of a hall sensor it uses induction. This is the possibility I lean towards the most since optical encoders and magnetic/inductive encoders have different advantages and disadvantages so it doesn't make too much sense for one to supplant the other. Magnetic and inductive encoders, on the other hand, seem to share most of the same advantages and disadvantages, but induction sensors are more complex (and therefore expensive) than magnetic sensors. Though I suppose they are immune to demagnetization.