As part of the FreeBSD powerpc* flag day (1300070), the auxv numbering was changed to the common format.
See D20799 for more details on that change.
While the kernel and rtld were adapted, libc was not, so old dynamic binaries broke for reasons other than the ABI change on powerpc64.
Since it's possible to support nearly everything regarding old binaries by adding compatibility code to libc (as besides rtld, it is the main point where auxv is digested), we might as well provide compatibility code.
The only unhandled case remaining should be "new format libraries that call elf_aux_info() which are dynamically linked to by old-format binaries" in my reckoning.