Apple iPhone TTY Adapter lets you use your iPhone with standard TTY devices, compatible with all iPhone models. Get fast, free shipping when you shop online.
From what I can tell it has been out of stock since roughly 2019, but the page is still up. I fell down this mini rabbit hole starting from this page.
The iPhone 7 and up, it became built into the phone without requiring a specific adapter. I don't know enough about TTY to know if that means you plug it in with lightning, or if the lightning to 3.5mm adapter also includes TTY functionality. :(