The "mda" audio processing plug-ins were available for many years as closed-source freeware from Maxim Digital Audio. They were developed by Paul Kellett, and open-sourced to SourceForge in 2015.
This repository is based on the code published on SourceForge.
These plug-ins were written using the original VST 2.4 SDK from Steinberg Media Technologies GmbH, which is no longer available, and Steinberg has ceased issuing licenses for it. As a result, this code is only directly usable if you happen to have a copy of the SDK, and can only be distributed if you happen to have secured a license from Steinberg prior to October 2018.
However, all of the code is simple enough that porting it to newer plug-in SDKs should be quite simple.
The updated versions of the mda plug-ins, compiled from the code in this GitHub repo, are bundled with Unify, a product developed by Shane Dunne & Associates, Inc. and distributed by PlugInGuru. Shane Dunne & Associates, Inc. is licensed to distribute VST 2.4 compatible plug-ins.