It seems as though the deduction theorem can fail in natural language, if we think of a valid inference as one that preserves certainty, rather than truth. What I mean is that if we are certain of (for instance) "If a German wins, it will be Schmitt," we will also be certain of "If the winner isn't Schmitt, it won't be a German"; but in general we're not obliged to be certain of "Either it's not the case that if a German wins, it will be Schmitt, or else if the winner isn't Schmitt, it won't be a German." (That is the material conditional corresponding to the inference.)
My question is whether this has been discussed in the logic literature. I haven't yet found anything in work by Ernest Adams and Vann McGee, but I haven't looked at everything by them, and of course the literature on the logic of conditionals is massive. Wondering if this rings any bells.
I would be happy to discuss the claim of deduction theorem failure on the merits, but my main question is whether it has already been discussed in print.
EDIT to clarify: By the deduction theorem, $\ X \vdash Y$ implies $\; \vdash X \supset Y$, where $\supset$ is the material conditional, so that $\ X \supset\ Y$ can be rewritten as $\; \sim X \; \vee\ Y$. Here, I am supposing that the logical consequence relation $\vdash$ tracks with a semantic relation, $\vDash$, such that $\ X \vDash\ Y$ means that whenever $\ X$ is certain, so is $\ Y$. That is what I mean by defining valid inference in terms of certainty-preservation.
For sentences containing the natural-language conditional connective $\rightarrow$, which is not the material conditional, certainty can be defined numerically (the details are in the writings of Adams and McGee), but the point of my question is that we have enough of an intuitive sense of it to make judgments about it in simple natural-language examples such as the one I gave. In that example, $\ X$ is instantiated as $\ A \rightarrow B$, where $\ A$ is "A German wins" and $\ B$ is "Schmitt wins," and $\ Y$ is instantiated as the contrapositive of $\ X$.