It seems that std::not_fn should reject the following code according to [func.not.fn]:

Mandates: is_constructible_v<FD, F> && is_move_constructible_v<FD> is true.

godbolt:

#include <functional>

struct OnlyCopyableFun {
  OnlyCopyableFun() = default;
  OnlyCopyableFun(const OnlyCopyableFun&) = default;
  OnlyCopyableFun(OnlyCopyableFun&&) = delete;
  bool operator()(auto) const;
};

int main() {
  OnlyCopyableFun f;
  auto nf = std::not_fn(f); // only ill-formed in libc++
}

Feel free to correct me if I've missed something.
Not quite sure why these standard call wrapper factories (std::bind_front, std::bind, etc.) require all argument types to be move-constructible, although the call wrapper produced in the above example is still move-constructible as its underlying type has a copy constructor.
It seems to me that just requiring is_constructible_v<FD, F> is enough, which is what the range adaptor object does.