Similar to #571, I've met this deprecation warning (which treats as an error in Unreal), when building the current latest release 5.1.1 source code with the newly released XCode 14.3.

Unreal Engine 5 build C++ with C++17 by default, so the original fix for C++20 is not enough.
Though Unreal Engine 5 didn't specify the default C standard, Clang has its default to C17

By default, Clang builds C code in GNU C17 mode

Thus, the commit introducing the deprecation will be triggered when compiling MiMalloc.c in the Clang's default C17 mode.