OBS Studio has a very small registry footprint. All user configuration (scenes, sources, audio/video settings, profiles, stream keys) is stored in %APPDATA%\obs-studio\, not the registry.
The portable mode (extracting the ZIP release) creates no registry entries at all.
Virtual Camera driver (OBS-Camera) registers a DirectShow filter under HKLM\SOFTWARE\Classes\CLSID\ and a kernel driver under HKLM\SYSTEM\CurrentControlSet\Services\OBSVirtualCam when the Virtual Camera feature is installed.
Browser source (CEF) and other plugins do not add additional top-level registry keys.