Updated list as of UE 4.21:
MagicLeap
AppleARKit
FGoogleARCoreXRTrackingSystem
FGoogleVRHMD
PSVR
OSVR
SimpleHMD
SteamVR
OculusHMD
WindowsMixedRealityHMD
Updated list as of UE 4.21:
MagicLeap
AppleARKit
FGoogleARCoreXRTrackingSystem
FGoogleVRHMD
PSVR
OSVR
SimpleHMD
SteamVR
OculusHMD
WindowsMixedRealityHMD