Carbon steerer tube scratch (inside the tube) - how bad is it?

Old question. Not sure if anyone is interested, but here is a shot. Qualifiyer: I am not a bicycle frame Designer or bike repair person; however I have spent 30 years in the aircraft industry telling people how to repair composites.

That picture is a bit misleading, looks like a flash blew out some color & detail.

The tear line & location makes me think this is a fiberglass isolation ply. If it is, the floppy piece would be white. Delaminated fiberglass is normally white, bonded is nearly translucent. The main use for a thin fiberglass isolation ply would be to prevent galvanic corrosion between the carbon & aluminum. The aluminum will eventually go away via corrosion, again white. I am assuming the head stem is aluminum.

Also, fiberglass has a 3X modulus of elasticity over carbon. It's more stretchy. Probably making it more resistant to the abuse from your expander plug & putting that extra layer between the plug & carbon.

I see no Structural issue here. a thin coat of room temperature curing resin, paint or even fingernail polish just over the missing fiberglass with a very slight overlap will take care of any potential galvanic corrosion issues. If not, it will take a long time for the corrosion to do much damage.