The CRYENGINE C++ API reference for the most recent version of CRYENGINE is available here:
C++ API for CRYENGINE 5.5
C++ API for CRYENGINE 5.4
C++ API for CRYENGINE 5.3
C++ API for CRYENGINE 5.2
C++ API for CRYENGINE 5.1
C++ API for CRYENGINE 5.0