Pretty sure its $HOME/.config/kwinoutputconfig.json. However it is a bit tricky now, in order to solve a bug regarding shutdown. I believe kde started storing this file in memory or caching it somehow, so if you try to manually remove it or edit it. It won’t work. It will still remember the previous contents.
When I want to remove the file now. I create a new empty file in its place and then use sudo chatrr +i on the file. Then reboot. Then do sudo chattr -i on the file. What this command does is make the file immutable so it can’t be changed. This prevents it from being overwritten with the previous contents.