A project switched to MPI-3 after realizing runtime crashes in very large simulations were due to MPI-2. The standard MPI-3 interface came out in 2012. MPI-3 is well supported now by compilers/libraries. Most of our users use OpenMPI or Intel MPI.
A limitation became apparent with Clang and MPICH, which doesn’t build right now due to Clang not yet having ISO_Fortran_binding.h and Flang being not yet ready for public use. GCC + MPICH or Clang + OpenMPI and many other combinations work. Thus macOS users who desire MPICH with Fortran MPI-3 bindings need to use GCC and build MPICH themselves.