While third-party dongles are available, the good news is that if you'd rather not pay for something unofficial, Apple has extended its MFi program where it will now cover third-party Lightning to 3.5mm cables, assuming that the manufacturers are able to meet the specifications laid out by Apple.
These news specs in no way suggest that Apple is giving up on Lightning.
USB-C receptacles will have other advantages such as enabling accessory manufacturers to make a single product which can be sold to Apple and non-Apple users both who might use the Type-C USB interface but not take advantage of the Lightning cable. This is the only way to connect an iPhone to one of Apple's USB-C laptops. Included in Apple's documentation for the news specifications are speakers and battery packs as products for which it could be advantageous to use a USB-C receptacle. Unlike with Lightning receptacles, Apple does not allow the port to be used for passthrough charging or sync of an iOS device. Products are also allowed to bundle USB-C cables with the MFi accessories, but manufacturers can opt to not include a cable or adapter and reduce their costs and or price in the process.More news: Where to get deals for National Pi Day
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The fact that now there is the possibility of buying this kind of third-party adapters from the MFi series means that cheaper alternatives will come soon enough too, to the joy of those who still use wired earphones with a 3,5mm jack with an iPhone. Instead, the headphone jack end of the dongle will be what's called a male port.
Apple introduced the specs into its MFi program at the beginning of the year, and it appears there aren't yet any products on the market taking advantage of them.