As has been discussed in the https://gcc.gnu.org/ml/gcc/2017-02/msg00041.html https://gcc.gnu.org/ml/gcc-patches/2017-05/msg01227.html https://gcc.gnu.org/ml/gcc-patches/2018-04/msg00810.html threads and in https://gcc.gnu.org/PR81084 the powerpc*-*-*spe* support which has been split into a separate backend hasn't been given enough maintainance and so is deprecated in GCC 8.
It can still be built with --enable-obsolete. If the maintainers don't manage to remove the dead parts of the port and start actually maintaining it early in the GCC 9 stage1 development, the port will be removed.
