P3372R3 states:

Notes for wording changes

Mechanically mark all functionality in [container] constexpr with exception of node-handle::mapped() (which is subject of CWG-2514). Also make sure all iterator types are constexpr.

However, in the wording of that paper, a notable omission is the constructor explicit map(const Allocator&), which is not marked as constexpr:

I believe this is an editorial issue.¹