I tried asking libffi-discuss@sourceware.org, but didn't get a response, so I'll try here:

Hi. I was wondering if arm64e was supported. And if not, are there plans for supporting it?

For the uninitiated, arm64e is the new architecture shipped with the iPhone XS and iPhone Xr.

https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/armv8-a-architecture-2016-additions
https://developer.apple.com/documentation/security/preparing_your_app_to_work_with_pointer_authentication

tl;dr: it signs the high-order bits of pointers for security. Code compiled for arm64 does not run on arm64e devices.