Maybe they are referring to Effective Modern C++ book by Scott Meyers.

I think "modern" is used to contrast newer C++ standards from C++98.

Or an older book.

"Modern C++ Design: Generic Programming and Design Patterns Applied" by Andrei Alexandrescu

Actually, he coined the term. Then Scott Meyers reused it, and now it is a common adjective used before C++ books, libraries, tooling, and so on.