From cppreference.com
This header was originally in the C standard library as
.
Compatibility header. true, false and bool are
in C++, but they were not keywords in C until C23.
Macros
__bool_true_false_are_defined
(C++11)(deprecated)
C compatibility macro constant, expands to integer literal 1
(macro constant)
Notes
<cstdbool> is deprecated in C++17 and removed in C++20. Corresponding <stdbool.h> is still available in C++20.
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++11 C programs no longer need the compatibility macro
__bool_true_false_are_defined since C23 deprecated this
macro in C++