"We predict the OLED model won’t support fingerprint recognition, reasons being: (1) the full-screen design doesn’t work with existing capacitive fingerprint recognition, and (2) the scan-through ability of the under-display fingerprint solution still has technical challenges, including: (i) requirement for a more complex panel pixel design; (ii) disappointing scan-through of OLED panel despite it being thinner than LCD panel; and (iii) weakened scan-through performance due to overlayered panel module. As the new OLED iPhone won’t support under-display fingerprint recognition, we now do not expect production ramp-up will be delayed again (we previously projected the ramp-up would be postponed to late October or later)."
https://www.macrumors.com/2017/07/03/ming-chi-kuo-iphone-omit-touch-id/