Why is std::shared_ptr::unique() deprecated?

Consider the following code:

// global variable
std::shared_ptr<int> s = std::make_shared<int>();

// thread 1
if (s && s.unique()) {
    // modify *s
}

// thread 2
auto s2 = s;

Here we have a classic race condition: s2 may (or may not) be created as a copy of s in thread 2 while thread 1 is inside the if.

The unique() == true means that no one has a shared_ptr pointing to the same memory, but does not mean any other threads have no access to initial shared_ptr directly or through pointers or references.