From cppreference.com
~unordered_multiset();(since C++11)
(constexpr since C++26)Destroys the unordered_multiset. The destructors of the elements are called (in unspecified order) and the dynamically allocated storage (if any) is deallocated. Note, that if the elements are pointers, the pointed-to objects are not destroyed.
Complexity
Linear in the size of the unordered_multiset.