Where is macOS 10.14 Mojave com.apple.Safari.plist ResponsiveDesignCustomPresetConfigurations?
The file is in /Users/$USER/Library/Containers/com.apple.Safari/Data/Library/Preferences/com.apple.Safari.plist
(replace $USER with your username).
However to edit it you have to disable SIP. Reboot and hold Ctrl+R, open Terminal and type csrutil disable && reboot.
Login and edit the file (either manually or using defaults command).
Reboot again holding Ctrl+R and type to the terminal csrutil enable && reboot to enable SIP.