I just ported my project from UE4.27 to UE5 (ue5-main branch) and the World Parition and One File Per Actor menus are missing. The documentation shows menus that are just not there: One File Per Actor | Unreal Engine Documentation
To add World Partition to a level, believe you still need to use the commandlet or start from one of the open world level templates. World Partition should also turn on OFPA by default for that level.