Partial template specialization of std::atomic for smart pointers

Background

Since C++11, atomic operations on std::shared_ptr can be done via std::atomic_... methods found here, because the partial specialization as shown below is not possible:

std::atomic<std::shared_ptr<T>>

This is due to the fact that std::atomic only accepts TriviallyCopyable types, and std::shared_ptr (or std::weak_ptr) is not trivially copyable.

However, as of C++20, these methods have been deprecated, and got replaced by the partial template specialization of std::atomic for std::shared_ptr as described here.

Question

I am not sure of

1. Why std::atomic_... got replaced.
2. Techniques used to enable the partial template specialization of std::atomic for smart pointers.