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.
However this is for a 3.5mm output cable, which means that this is more for users to plug their iPhone into something, as opposed to plugging headphones in. For example if you wanted to connect your iPhone to a speaker or HiFi system that does not support Bluetooth, using this dongle is one way to go about it, especially since we doubt there are many Lightning to 3.5mm aux cables lying around.
This means that users have to rely on the bundled Lightning to 3.5mm input cable, plug in an aux cable, and plug that cable into the speaker. This new spec means that manufacturers could create a more streamlined experience. Exactly how many users will need to take advantage of this is unclear, but the option is there.