World Partition is built on top of the level streaming system, which means it has the same capabilities and several quality-of-life improvements over the traditional level streaming setup. Could you share which version of UE you last used WP with and what the issues were, in case there are some improvements we could make around the feature?
As for why the editor loads the streaming levels by default, this predates my time with the engine, and I’m not entirely sure of the exact reason. As for unloading streaming levels in the editor, you would need to investigate this since it’s not something I have explored. At a glance, I see in ULevelStreaming::DetermineTargetState that going from ELevelStreamingState::LoadedNotVisible to ELevelStreamingTargetState::Unloaded through setting bShouldBeLoaded only works in game worlds. It’s been a long time since I’ve taken a look at World Composition, but it allowed unloading levels, which it did by completely removing the streaming level from the world (see FLevelCollectionModel::UnloadLevels) rather than setting the streaming level as unloaded while still keeping it in the world’s list of streaming levels.
-Ryan