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.
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.