Update regarding the auto-retopology subject:
@norgeotloic has released a free add-on called BakeMyScan, which includes a number of auto-retopology options. Note: it’s necessary to separately install executables of the auto-retopology tools, such as Instant Meshes.
Thanks Metin for linking the thread!
To anyone interested: I am not sure about distributing the executable files with the addon, but be sure that if BakeMyScan gains its little popularity, I’ll definitely contact all of the developers behind the software I interface to have a chat with them on this matter. I’ll definitely contact them anyway, I just need some 48 hours-long days 
Also, and except for Quadriflow at the moment, “manually” installing really means downloading, and selecting a .exe file in your user preferences.
Hope you’ll enjoy !
Good point. I’ve changed “manually” to “separately” in my post. 
A fresh update:
@Albertofx has updated Dynremesh to 2.0, which is Blender 2.8 ready, and includes an additional QuadriFlow add-on called Autoflow. I don’t know if QuadriFlow is already compiled for macOS.
This is a great thread!
Any plans to bring it to May/June 2021?