The corresponding `std` types were removed before C++20 was finalized:
https://wg21.link/P1959R0.

They are unused. The language mechanisms and recommendations changed since they
were originally proposed. In particular:

* An explicitly defaulted `operator==` is defined in terms of memberwise
  `operator==` rather than sibling `operator<=>`.

* An `operator!=` can be implicitly added to an overload set in terms of
  `operator==` rather than `operator<=>`.

* A class which has equality but not ordering defined should provide
  `operator==` rather than `operator<=>`.

PiperOrigin-RevId: 587834267
Change-Id: I2c2513c13f3485b9edc6a345dca4a577d8e65167