Yes. In some competitions about ten years ago in ex-USSR it was typically Windows + Far Manager (maybe some other stuff, but not always), in international competitions it was some flavor of Linux with a stock KDE (hence Kate) or GNOME (hence Gedit).

No professional IDEs almost certainly (no license to use), and the set of free IDEs depends on what organizers like: some installed NetBeans, some installed Eclipse CDT, some did both, some did neither.

No external resources like configs and plugins are typically allowed either.

Of course, it depends on a competition. I'm mostly talking about IOI (International Olympiad in Informatics for school students) and ICPC (International Collegiate Programming Contest). Online competitions are another beast.