I appear to be also having this issue. I recently bought a headphones adapter-splitter that adapts the USB-C port on my iPhone 15 Pro to a 3.5 mm TRS audio ("aux"/auxilliary/headphones port) plus a USB-C port, to output analog audio in Audio Accessory Mode and accept charging at the same time. The vendor says it should work with iPhone no problem, but I suspect that maybe the same change you've noticed with iOS 26 has made that no longer true.
No matter what solution I try, like the several approached mentioned in USB C Headphone lead on iPhone 15 SOLVED … - Apple Community , I cannot get the iPhone to output plain old regular analog audio from the USB-C port. I also cannot get it to charge using the new splitter. The phone normally prompts me when I connect accessories to the USB-C port, but never prompted me for this splitter.
I have Lockdown Mode enabled, and I know that that affects how iPhone treats USB accessories somewhat, but in my experience I still get the option to use them. All except this splitter. Very frustrating!