Reading the iFixit teardown: https://www.ifixit.com/Teardown/iPhone+6+Teardown/29213#s69255 it would appear that the iPhone 6 doesn't have the usual NFC flat coil antenna. This would make sense if it's just designed to emulate a card for Apple Pay rather than energise tags. I'm guessing that they chose this instead of a standard flat coil antenna due to the metal back - a standard coil antenna wouldn't work very well behind a metal plate.

Given that it might be hard to energise a tag without such an antenna, I wouldn't put money on the NFC ring working if someone writes their own jailbroken app to drive the NFC chip, as the NFC ring is an order of magnitude harder to read than normal NFC tags (apparently this is a "feature" but it means I can't even use it with a case on my Nexus 7, and none of my other NFC readers can read it at all...).

So it's much more likely that there's no NFC API simply because it wouldn't be that much use rather than any more fanciful reasons.

https://www.ams.com/eng/Products/NFC-HF-RFID/NFC-HF-Booster/AS3923

The only thing I can see there is that the AS3923 states that it's designed for "mobile payment" so I wonder if it's really designed to be a RFID reader. I'd read the datasheet but you have to "request" it which really means you or I can't have it...

Reading the iFixit teardown: https://www.ifixit.com/Teardown/iPhone+6+Teardown/29213#s69255 it would appear that the iPhone 6 doesn't have the usual NFC flat coil antenna. This would make sense if it's just designed to emulate a card for Apple Pay rather than energise tags. I'm guessing that they chose this instead of a standard flat coil antenna due to the metal back - a standard coil antenna wouldn't work very well behind a metal plate.

Given that it might be hard to energise a tag without such an antenna, I wouldn't put money on the NFC ring working if someone writes their own jailbroken app to drive the NFC chip, as the NFC ring is an order of magnitude harder to read than normal NFC tags (apparently this is a "feature" but it means I can't even use it with a case on my Nexus 7, and none of my other NFC readers can read it at all...).

So it's much more likely that there's no NFC API simply because it wouldn't be that much use rather than any more fanciful reasons.