ATKAutoSwell is a free compressor VST, VST3, Audio Unit plugin developed by Matthieu Brucher. Compatible OS(s): Windows 64b, macOS.
ATKAutoSwell works like a compressor, but instead of only acting on the signal above the threshold, it applies the gain transfer function to the full signal. So above the threshold, you can lower the output signal.
http://www.audio-tk.com/