[Brightcolours]

It must be wonderful to live in a problem-free Canon world. That's the advantage of not having the resolving sensors for quite a while, the little vibrations remain just invisible. However, this lovely dream could come to an end

 

I think, anywhere shutters are involved and careless damped - which was also for Sony's mirrorless α7R first gen. a problem - one needs to take care instead of hoping this is only an issue for other people's systems.

If you don't want to place things into perspective.....

 

I will do it. 

 

Horsack tested 4 things:

 

1. Shoot on tripod with ballhead. No mirror lock up.
   
2. Shoot on tripod with ballhead. Mirror lock up with 2 second delay. 2 second delay is too short, as anyone with some knowledge knows.
   
3. Shoot on tripod with ballhead. Live view with EFCS.
   
4. and 4: shoot 6D on tripod with 3 brackted shots quickly after eachother, and with 1 second delay. It showed that on (his) tripod shooting with quick bracketed shots only the 1st image is totally sharp, and that 1 second delay is enough to counter that.
   

He did not shoot normal handheld, where vibrations get absorbed mostly. The vibrations without MLU are expected, we knew this in the days of film already. That the 2 second delay is a bit short is also well known.

So, did Horsack's tests show a mirror slap/shutter slap issue? No. As you yourself have noticed, you do have one with a D810/800 and for instance a 300mm f4 VR PF. And as others have noticed, there is a shutter slap issue with a Sony A7R. And a number of Olympus cameras which have IBIS. I have read some reports that state that some Pentax cameras also have a shutter slap issue (also IBIS).