I think it was a bit of a misnomer b/c it sounded like some glorified style guide. But it was actually a set of tools and helpers/wrappers that effectively made it impossible to write a lot of unsafe pre-modern C++.
For whatever reason it's never really caught on, though it looks like it's still being worked on: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines
In principle it seems to kinda strikes the right balance where the language is narrowed - it's still all valid C++ (not an entirely new thing), but not all C++ is valid under the Core Guidelines
I'll be honest - I never tried it myself so I don't know it's deficiencies. Maybe someone who's given it a good try could weight in