As of N4910, I observe that [stacktrace.syn] doesn't depict #include <compare>, but [stacktrace.entry.overview] specifies

friend constexpr strong_ordering operator<=>(const stacktrace_entry& x, const stacktrace_entry& y) noexcept;

This appears to have been a simple race condition with LWG-3330 "Include <compare> from most library headers". The LWG issue was accepted in Feb 2020 before <stacktrace> was accepted in Nov 2020.

@jwakely, @CaseyCarter, and I believe that the LWG's intent is very clear, and this is a very simple issue, so we hope that it can be resolved editorially without the need for a separate LWG issue.