!kidSam kidnapped by a shapeshifter and replaced by a doppelganger FOUND
I read this fic years ago, so I'm not sure if it's not deleted. It was on AO3. The story involved corporal punishment that was borderline abusive because the shapeshifter was messing with families. Basically, that creature would kidnap kids that are most loved in the family and influence parents in such a way that they start being abusive towards less treasured children. Dean started being abused. Then the only way to get rid of the influence is to let the parent beat the shit out of you, basically. Dean provoked John and returned Sam.
If anyone interested, here’s the link: https://archiveofourown.org/works/768161