I think the HN title is incorrect, the title in the linked post just says C++17, not C17. Likewise, the JEP doesn't really mention C other than in passing, and the OpenJDK code I've looked at myself is all written in C++, so AFAIK this is really just a change from C++14 to C++17.

As for the differences between C++14 and C++17, the things I use most often are consexpr if, variable declaration in if statements, std library features like std::optional and std::string_view, lambdas got more powerful, and probably a bunch of other things I'm forgetting. There are a ton of "What's new in C++17" articles you can find online if you want more information.