From memory I believe 19c is the last supported version but I cannot find the compatibility page to double-check.  You will also need to be on Apps9 UPD16 for 19c.

Here’s a screenshot I found from Nov 2021 - which is well after Apps9 went into Extended Support - showing 19c was the newest supported version at that point, and I don’t believe that IFS will have certified an IFS version under Extended Support on a newer Oracle version:

Nick

Oracle 19c is supported by Apps 9 UPD16 and above.

Attached is the latest Apps 9 Support platform guide.