Whenever an art jam becomes inactive and turns to read-only, only the owner of the art jam has access to it. I propose you should allow people who own a layer to decide what they want to do with the layer.

For example, I was drawing with others in an art jam; a day later the canvas became read-only but my drawings remained viewable. I didn't want anyone to screenshot and upload my art elsewhere so I'd like to delete my layer from the art jam. Unfortunately, I need the owner to make the art jam active again to do so - but I don't have a means to contact the owner of said art jam.

To re-state, you should allow people who own a layer in an inactive, read-only, art jam to decide whether it stays or can be deleted.