I'm not aware of any plugins, but in case anyone is curious about how to replicate that setup on Linux systems with PulseAudio, you can create virtual outputs with:

    pactl load-module module-null-sink \
        sink_name=Device_Name \
        sink_properties=device.description=Device_Name

If some program filters out output monitors from its input list, you can usually use pavucontrol to force-change it. Or, you can create a linked virtual source:

    pactl load-module \
        module-virtual-source \
        source_name=VirtualMic \
        master=Device_Name.monitor \
        source_properties=device.description=VirtualMic

Then in OBS settings you can set up the virtual device as Monitoring Device.

I took the commands from one of my side projects, maybe someone will find other parts of it interesting: https://github.com/pzmarzly/mic_over_mumble