Just like legacy Actions get a slot "Legacy Slot" when they're versioned, their layer is now also called "Legacy Layer" to be consistent with that. When upgrading a legacy Action to a slotted Action, it also gets a layer and a keyframe strip on that layer. Those are not shown in the user interface yet, but will in the future. Pull Request: https://projects.blender.org/blender/blender/pulls/133009