Haven't been able to 100% confirm, but from the game that I played with him and the last game I spectated with him in it, Static Link's buff overlay says 'Stealing 0 attack damage per second' even though the buff icon still has it increasing. I cannot remember if the actual damage in the stats box goes down as well, but that it does not and heroes still seem to do damage after a full Static Link (level 4) when they do not have a high enough damage output to overcome the negative damage they get.