It's completely idiotic because internal door locks, the kind in the knob, have a little hole that you can use a tool to unlock it from the outside. I think it's meant for in case of emergency, but the point is they're trivially easy to unlock.

The parents are more likely control freaks.

a window was open in the house and I accidentally shut the door too loudly one time and my door was removed for ~~weeks~~ months (there was no history of me slamming doors, I think punishment for the sake of punishment was the point)

no privacy for changing, sleeping, etc. - it was stressful

EDIT: I just remembered it was more than weeks, it was months - I had journal entries about wondering when I would ever get my door back.

Did they also punch a hole in the wall when it was hot instead of opening a window? Locks can be removed from doors.

Yea my parents did that. I didn't want to spend time with them because they were abusive assholes (why yes, CPS were called!), but apparently that meant there was something wrong with me, so they took the door off my room so I couldn't have a safe space from the constant screaming and violence. Guess who swung hard Trumpy antivaxx Q believers in 2016?

My parents just ensured the lock on the door knob had a slit on the outside so you could stick anything in there and rotate to unlock it. Half the time, they just used a long fingernail or the edge of a coin. It didn't require much effort.

They'd still get mad at me for locking my bedroom door and would threaten to remove the door if I kept locking it, but it took them maybe 10 seconds to open it without tools.

They never followed through on the threat; I kept my bedroom door all throughout my childhood.

My parents did this and claimed it was normal and to "ask your friends at school" I did, and they said it was abuse. She didn't change her mind.

Yes but this is actually a thing parents do

If you did then also give her training so she doesn't take her own limbs off, or worse.