Linux PPC64le requires Power8 since the beginning.
AIX requires Power8 with the new OpenXL based build (JDK-8307520). The old build has been removed in JDK 23 (JDK-8327701).
Linux PPC64 Big Endian is no longer officially supported (only kept alive for development, debugging and testing purposes).

The following checks for old processors are no longer needed:
8: VM_Version::has_lqarx()
7: VM_Version::has_popcntw()
6: VM_Version::has_cmpb()
5: VM_Version::has_popcntb()
These ones and some more checks for old instructions are no longer needed. All code which is no longer reachable when removing them should also get removed.
Checks like "PowerArchitecturePPC64 >= 8" (or older) can be removed.

Atomic::PlatformCmpxchg<1>::operator() can be simplified by using sub-word instructions (lharx, lbarx).

Temp registers can be removed from cmpxchgb and cmpxchgh.

os_aix.cpp contains code for old processors: PV_7, PV_7_Compat, etc.

Build flags "-mcpu=powerpc64 -mtune=power5" for Big Endian linux should get replaced by "-mcpu=power8 -mtune=power8" as already used for linux PPC64le.