Interesting!
I'd be curious as to what types of bugs were caused by these implicit casts..
Note 8.3 was in the days back before ORMs became popular, so "just write better SQL" was a perfectly decent solution to the problem back then. Now days, this requirement might make Postgres incompatible with certain ORMs out there, which is a bummer. I'm wondering if these ambiguities you speak of could be solved in other ways. Such as implicitly cast iff the intention is not ambiguous, otherwise raise some sort of "ambiguous" error or default to some behavior.
Mike