this is a missing feature in Rhino 8.
in Rhino 7, you can organize linked blocks and attached worksession files into layers. it doesn’t remember when you close the file, so it was never great but at least you can put them into a parent layer so it doesn’t clutter your layer pallete. You can drag them up or down as you like as well.
In Rhino 8 you can’t drag these at all or collect them in a parent layer to turn on/off with one click.