That is a very good point.
I have added a section which discusses the ABI implications of this
proposal, based on my limited knowledge. You know tremendously more
about libstdc++ than I do. Is my understanding that libstdc++ doesn't
have to take additional compatibility measures correct?
We would have to keep exporting the original non-inline definition for code already linked to an older version and expecting an extern symbol in the library. The new headers would define the function inline, calling something like your __uncaught_exceptions_impl example. That's technically an ODR violation, because there's an inline definition in the header and another non-inline definition in the library. It works though, because of reasons. The implementation can just declare "this doesn't violate the ODR" :-P
The __uncaught_exceptions_impl could even be an alias for the non-inline uncaught_exceptions definition (or vice versa) to avoid code bloat.
So I'm not concerned about the difficulty of implementing the proposal, but it's definitely worth mentioning.