I'm working through updates to vector spec, and one part of clean up
is changing text where it has mandatory raising of illegal instruction
exceptions on unsupported encodings to instead state the encoding is
"reserved". This will allow future extensions to use these encodings,
while current implementations can continue (and are encouraged) to
raise illegal instruction exceptions on unsupported reserved
encodings.

We didn't talk through this in meetings, but I assume this will be
uncontroversial.

https://github.com/riscv/riscv-v-spec/commit/96a9bc96640b9a12bf8868634671e501f2a24b77

Krste