But the MPI standard says this is valid and can be used to avoid the need to keep track of ISEND request handles. It is clear that if the matching RECV has completed then the unaltered content of the ISEND buffer is no longer needed. The RECV cannot have completed unless the message got out.

I tend to agree with this sentiment, but the MPI standard has long had a prohibition on touching the transmit buffer, because you never know what the implementation is using that buffer for. So... anything that ever had a reliability protocol and did zero copy has had this issue.

Erez is pointing out a case where the semantic stipulated by the MPI standard back before RDMA adapters were available becomes flawed with the way RDMA adapters now work. The content of the ISEND buffer is irrelevant once the destination RECV has completed but some other things about the ISEND buffer are still in limbo

I don't believe this is new. Anything that ever provided a reason that you couldn't touch the send buffer likely had a similar issue. Keith