However, few people actually use Pro*C nowadays. I spent almost 10 years working for Oracle, and I never once came across anybody using it. I think almost all use of it is in legacy applications which date back to the 1980s, first half of the 1990s at the latest. Obviously still enough use for Oracle to keep on supporting it, even with some occasional minor enhancements, but rather fringe all the same.
I don't think many people use ECPG either. I'm sure some people must, but I myself have never seen it. Probably most use is in porting existing applications from Oracle Pro*C, or its equivalents such as Informix E/SQL. Actually, it is interesting to observe that the section on ECPG's Informix compatibility mode is a lot longer than the Oracle equivalent. I don't know if that's because less compatibility is required (maybe Pro*C is closer to ECPG already), or if that's because Informix compatibility has received more investment.
I've used pro* C for a couple of things (in banking and utilties, 1990s) where people wanted interactive batch functionality for something more complex than you'd generally put in a shell script. I think most of the pro* C usage came from pre Oracle 7 when PL/SQL didn't exist.
I don't know where to find it on their website, but the docs RPM that they provide bundles the PDF.