According to Jennifer Bailey, vice president of Apple Pay and Apple Wallet, a digital version of the U.S. passport will "soon" be available on iPhone.
the feature is coming soon
During her keynote at the Money20/20 USA conference, Bailey confirmed the upcoming feature. Apple's website states it will be available "later this year" but no longer mentions the need for a software update.
Author's summary: Apple iPhone to get digital U.S. passport feature.