Apple is said to have plans to bring Touch ID back to iPhone. The under-display sensor would be built by Qualcomm. 2020 is pencilled in as the release year.
A new report by the Economic Daily News (via MacRumors has Apple representatives meeting with Taiwanese company GIS to discuss the possibility of putting an under-display fingerprint sensor into a 2020 iPhone.
The report - or pair of reports, to be accurate - claims that Apple intends to use a Qualcomm ultrasonic fingerprint sensor in at least one of the iPhone models that are pencilled in for a 2020 release. However, the same reports note that the design could be pushed back to a 2021 iPhone instead. That, if anything, sounds like the hedging of bets to me.
As MacRumors notes, ultrasnoic fingerprints ensors are just one option here.