when did apple discontinue 3.5 mm headphone to lightning

The Lightning to 3.5 mm adapter is still available. To the best of my knowledge Apple never sold an adapter to go the other way.

Lightning to 3.5 mm Headphone Jack Adapter - Apple

The adapter is still available.

https://www.apple.com/ca/shop/product/MMX62AM/A/lightning-to-35mm-headphone-jack-adapter

I believe they stopped including one with the iPhone 10.