DEPR: deprecate msgpack support
closes #27084

I’m generally on board with this, but concerned about the fact that msgpack uses internals, so if this becomes “downstream” then that may make it harder to simplify things in internals if we are worried about breaking downstream. > I’m generally on board with this, but concerned about the fact that msgpack uses internals, so if this becomes “downstream” then that may make it harder to simplify things in internals if we are worried about breaking downstream.

that’s all the more reason to deprecate this actually. 