It is not something new in 5.0, as you can see on the Group Input nodes to which those double lines are connected, there are these gizmo icons next to the sockets indicating that these values are editable with the gizmos you can see on the object:
The gizmos have already been added to Geometry Nodes in version 4.3, the double lines are there to show that information travels both ways - from the gizmo to the nodes but also the other direction if you have editable input fields in the modifier.
So for example when you move the cross gizmo in X direction, the Offset changes. But you can also manually enter an Offset value in the modifier and then the gizmo moves with the value you have set there. The reason something becomes a single line means in the example of the Switch node that there is no information going from the False socket to the gizmo, the Switch node is just receiving data. The lines stay double until they get plugged into a not-gizmo node where they terminate (or something like that).
Something more about gizmos in Geometry Nodes you can find in my answer here: What do I need to use the Transform Gizmo in Geometry Nodes?, but they can become a bit complicated so if you want to learn more about them I would recommend you watch some dedicated tutorials.
