I explained in detail why this is simply not the case
I know of no way to provide errors in the current model. You can produce
cataclysmic failures with extensions, and you can produce silent errors
with header fields, but you cannot produce a warning.
It's just the opposite. In the current model an MTA can downgrade when the
next hop doesn't support the feature. The degree to which this is appropriate
depends on the particular feature, but the negotiation mechanism makes
this possible.
With the model you're proposing the destination has no idea what to do
with the feature. A generic warning isn't necessarily appropriate -
so specifying that behavior further limits the utility of the model.