REPORT
I installed this firmware update 2.2 on my 5SII and took some handheld high-res test pictures.
I had the handheld high-res feature on my GH6 and it never worked very well; my hands are not really steady and more often that not the picture would fail to process. That is a M43 camera with good stabilization so I did not have great expectations for the fiull frame 5SII. But surprise, the S5II pictures I took all processed. The question is, how good are they?
With update 2.2 there is a new high resolution setting; 'Handheld High-Res, ON or OFF'. When set to ON, Motion blur processing defaults to MODE 2, perhaps implying to expect somewhat poorer performance. Also with Handheld High-Res set to ON a new icon appears on the camera screen, the multi-picture icon is combined with the stabilizer icon - possibly implying that the stabilizer is synched with all the pictures. When you shoot the viewfinder goes blank for all the shots, so you don't know how stably you hold the camera.
I shot in RAW, in daylight. With the 16-35 f/4 lens, at 35mm, my results varied from good to outstanding. At best these were true 12000 by 8000 pixel shots. When blown up x200% or higher the pixels appeared uniform across sloping line features. At worst there was some camera motion that displaced a few pixels, and you could see this at x200% and higher.
With the 24-105 f/4 lens, at 105mm, the results were consistently outstanding. This lens has optical stabilization, combined with the IBIS.
So tentatively the handheld high-res results look good, and this is a feature I'll use. It seems better when optical stabilization is used. With any handheld shooting your results will vary, so I'm anxious to hear what others find.