Apple supports STIR/SHAKEN checkbox in iOS 13
The Apple iOS 13 features list includes a feature for “carrier-verified calls.” This feature will indicate calls that have been verified by STIR/SHAKEN.
Here’s how Apple describes this feature:
A checkmark is displayed next to calls in the recents list when they are verified by the carrier, so you can be sure that a calling number is accurate and has not been spoofed. Supports the STIR and SHAKEN standards.
Note that this only mentions the recents list and does not mention the call answer display. It will be interesting to see how this develops.