Quote from https://developers.redhat.com/blog/2016/02/29/why-cstdlib-is-more-complicated-than-you-might-think/
By the mid-2000s, a carefully written C++ program could use the common intersection of conforming and non-conforming C++ implementations and be mostly portable between implementations. Such code needed to consistently use std:: to qualify names after including a <cxxx> header, and consistently not qualify names after including a <xxx.h> header. But in the real world, developers aren’t always so careful. Even if they are aware of the precise requirements of the standard, it’s easy to accidentally use strlen (without qualification) after including <cstring>, and if the code compiles then it will get shipped. This meant that millions of lines of C++ code were unwittingly relying on properties of their implementation which violated the standard, creating a serious portability problem.

Quote from src/core/scanner.cpp

void Scanner::scriptMessage(MessageLevel level, const char *error, ...) const
{
        /* ... */

        sprintf(newMessage, "%s:%d:%d:%s: %s\n", d->scriptIdentifier.toUtf8().constData(), 
currentLine(), currentLinePos(), messageLevel, error);
        va_list list;
        va_start(list, error);
        if (messageHandler)
                messageHandler(level, newMessage, list);
        else
                vfprintf(stderr, newMessage, list);
        va_end(list);

        /* ... */
}

Quote from src/core/scanner.cpp

#include <cmath>
#include <cstdio>
#include <cstdlib>

$ grep -r "using namespace std" src/ | wc -l
0

$ grep -r "std::printf" src/ | wc -l
0