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
2c92cfb687