A Y capacitor prevents the neutral from shorting to ground, in this case Apple has cleverly avoided the issue by not having a ground whatsoever.

I am however curious about your design for a galvanically isolated AC-DC power supply.

That's not what the Y capacitor is doing here. All Y means is "high voltage withstand, will fail open, and we have receipts". Here, it's not line-to-ground, but primary-to-secondary.

Isolated ACDC converters are very common these days. Companies like Traco will sell you modules: https://www.tracopower.com/isolated-power-supplies

You'd use X for L to L/N, Y is for N to Ground. Those isolated power supplies are definitley grounded, definitely not what Apple is using, and will probably shut themselves off if they lose their ground connection (or at least signal a fault to the operator)

Unlikely, since you can quite happily buy two-prong power supplies.

And most packaged modules don't even have ground connections, e.g. https://www.tracopower.com/sites/default/files/products/data...

You're also confusing the Y designation, which relates to the capacitor properties, to the application. Not all Y capacitors are used for line-to-ground applications. However any application where a failure to a closed state would result in a shock hazard must use Y-rated capacitors.