https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91886
--- Comment #35 from David Edelsohn <dje at gcc dot gnu.org> --- The ELFv2 ABI does not require Power8+ because the subroutine linkage convention requires Power8 instructions. It requires Power8+ because the ABI provide a minimum guarantee to the programmer / developer / toolchain / distribution of the minimum ISA that will be encountered. The ABI can function on earlier hardware, but the ABI asserts to the programmer that one can utilize the instructions available in Power8 without special accommodation or feature tests. The authors of the ABI (IBM) didn't want to provide a guarantee of backward compatibility to the beginning of time, which creates challenges for the minimum ISA level supported by, for example, a GNU/Linux distribution.
