From the proposal:
Although there is possibility to implement standard conforming invoke function template as a constexpr function, the proposed wording does not require such implementation. The main reason is to left it consistent with existing standard function objects, that could have such definition, like std::mem_fn, std::reference_wrapper and operator wrappers. Furthermore imposing such requirement will block the implementation of invoke that refers to std::mem_fn.
This proposal assumes that constexpr addition to the header would be applied consistently by a separate proposal.
Both constexpr and standard library based implementation are presented in Implementability section of the proposal.