I just wanted to add a quick bit of followup based on my experience today: In September, after this problem cropped up, I did a build using Corona 2019.3476 targeting iOS SDK 12.1 that worked fine. Today, the same build combo resulted in crashes on purchase on devices running iOS 9. However, after downloading the latest daily build (3545) and activating the new Apple IAP plugin, purchasing on iOS 9 is once again working.
So, if you are using an older version of Corona targeting an older version of the iOS SDK, you might start seeing transaction failures in future builds. Obviously, your mileage may vary.
Also, “Restore Purchases” is still a little crash-prone, but upon relaunch, purchases are successfully restored.
Thank you for getting this taken care of, Team Corona- I appreciate it!