Equipment for IFR outside CAS is barely legislated. Its more down to aircraft type certification.
Aircraft certificated under a modern code like FAR pt23 or CS23 may specify instruments, gyros, back-up systems that are required for IFR or VFR flight.
Vintage aircraft may historically retain their right to fly IFR with just an ASI, Alt and a big turn and bank indicator if that was the basis of their original certification.