Other than that, again, per that section's rules, not being a commiter myself, I'm in no position to approve such a change. I wonder, however, how "urgent" such a final change is to you and why a deprecation cycle does not fit your purpose (even though, admittedly, it may represent more effort).
I think that between the low likelihood that someone went crawling over the attributes manually, the RoI of having a deprecation cycle with some intermediate solution that later needs to be cleaned up, and the fact that this would be a clean break (i.e., "AttributeError") rather than some obscure error, I am at least interested in opinions about going the exception route.
ProcMon is non-trivial to product ionize, and I'm not aware of anyone even using it in production, other than me, let alone crawling over its internal state.