The real issue here is that Avast considers the Download folder to be some kind of temporary windows folder. It has never been this for users or for Windows itself. It is the default for Microsoft downloads as well as most other programs.
When I checked out the 'clean' feature and it gave me 75Gb I knew it was wrong. As a regular CCleaner user I expected a couple of gbyte at the most. Maybe they have it in there because the numbers look So Impressive.....
In most instances, once you've downloaded the installer and then installed a program, that file really is not needed any more.
Dome of us use that folder for more than just installation programs. If that's the case, then this is not a folder you want to empty.
I certainly would never allow that folder to be emptied.
To be fair, there are other clean up tools that also select the Downloads Folder as a folder to clean.
It is still up to the user to verify as to allow or not allow any cleaning function before it actually happens.