As recently as January 2024 I have been able to send videos via iMessage that were ~1.5GB. besides iOS updates the only thing that is different is that I recently got an iPhone 16 Pro and now it doesn’t work. I regularly use the developer betas.

The problem is with your headphones (I believe). I had been experiencing the same problem for several months but then saw on the news that Apple had extended the time for iPods to be returned under warranty for the same problem that you are describing. I made an appointment and took them in, the did some sort of test on them and sure enough found them to be defective and they replaced them for me (no charge).