std::stacktrace is almost std::vector<stacktrace_entry>. This makes it an obvious candidate to define an alias for std::polymorphic_allocator.

Daniel:

It should be pointed out that a theoretically possible (additional) template alias

namespace pmr {
  template<class T>
    using basic_stacktrace = std::basic_stacktrace<polymorphic_allocator<T>>;
}

— albeit it would seem a natural choice when comparing with existing pmr typedef additions — would not provide any advantage for the user, because template parameter T would essentially need to be constrained to be equal to std::stacktrace_entry.