A few versions of iOS ago, the stitching algorithm for panoramas was updated, which produces results that in my opinion look less good for what I'm using the panoramas for. I was exploring developing a custom panorama app but couldn't find the API for taking panoramic photos, much less stitching them. Is there an API in AVFoundation or elsewhere to use for capturing a panoramic photo and stitching it?
There is no API in AVFoundation for capturing panoramas. The Vision framework does contain APIs for image registration, which can be helpful in an algorithm that produces panoramas. See Aligning Similar Images for an example of image registration, but also note that expanding upon this sample to capture panoramas is beyond the scope of the support I can provide to you.
I also strongly recommend that you file an enhancement request (via Feedback Assistant) for an API that allows you to capture a panorama directly