This is surprisingly common. C++ is huge and filled with many features that are only understood by a small subset of folks and so many teams have restricted coding standards that define which features can be used and how.
Sounds reasonable. In a project I used to work in I disabled stdlib, exceptions, RTTI. Not sure what else to disable to essentially have C with different syntax.
extern “C” on everything, if you care about linking compatibility, gets you another chunk of the way there by also disabling overloading etc.