https://wccftech.com/sony-on-ps5-backward-compatibility/

From the looks of it, complete PS5 backward compatibility isn’t definitive yet as the development team is still verifying whether this can be secured.

Earlier this year, Sony officially confirmed the existence of a new PlayStation console, the PlayStation 5. As the console is partly based on the architecture of the PlayStation 4, backward compatibility with PS4 games was confirmed. However, in a new statement given to Japanese outlet Famitsu, Sony has said that, while the console has been designed to achieve backward compatibility with PS4 titles, the development team is currently hard at work to verify whether “complete” backward compatibility can be achieved.