I find it hard to believe they'd drop that, though. WAY too much Pro*C stuff out there.
I can give one example that is somewhat similar. Informix dropped support for their C-language API libraries in the early or mid 90's. (They had support for embedded SQL, but the people using their application language library were out of luck - except to scramble to convert everything to embedded SQL, another language, or another database.)
Granted, by that time Informix (once a leader in databases) was falling increasingly further behind Oracle, so their decision probably effected far fewer people, and I suppose most of their customers were probably using a 4GL rather than C, but at least one fairly major database player *has* done something like that.
Come to think of it, it is a bit odd that they got rid of an API interface entirely; I know DB2 has one and obviously Oracle still has one - at least for a while. Doing things like that may have been one reason they completely fell apart in the late 90's.