Thanks for testing, and my sympathy for your health situation.
I suppose mileage may vary for others, but "stunningly useful" is most definitely a description which accords with my own usage of the thumbnail browser. I too am very careful about how I organise and save my DVD backup disks. As Stephen will no doubt agree :) - I learnt the hard way that neat organization at an early stage can save so much hassle later when attempting to locate files. For me the process of organization has now become habitual, and commences at the download stage. I save all my new downloads into appropriately nested directories (on a partition which is used exclusively to house downloads whilst awaiting burning).
For example, saved files on all my 3D backup DVDs, now tend to be housed in an identical framework of directories. A typical DVD might contain thousands (certainly many hundreds) of 3D models, 3D clothing items, 3D props, 3D hair figures, 3D light sets, 3D animals, 3D pose files, 3D utilities, etc, etc, etc..... Similarly, within my 3D props directory there will most likely be nested sub-directories designed to house (say) vehicles, furniture, foliage, weapons, scenes, etc, etc. Again my vehicles folder will most likely contain further sub-directories relating to cars, motorcycles, boats, aircraft....whilst my furniture folder might house sub-directories of chairs, beds, drawers, tables, ornaments, etc. Usually each individual item is housed in it's own directory, and for many years now I've made sure that whenever I save (usually a zip file of) each item, I include alongside a JPG image which pictorially explains the contents of the saved file.
Say I'm working on a 3D scene, and I need to find a particular type of chair which will blend nicely into the scene. Using InsideCAT's thumbnail browser (prior to 4.11) it's been a pleasure to visualise all the thumbnails within my "chairs" directory, and often I end up using items which I didn't know I had! Alternatively I might on other occasions choose instead to visualise all the thumbnails in the parent "furniture" directory, and sometimes gain inspiration which I wasn't necessarily seeking! Stunningly useful indeed!
If you do decide to report a 'bug' Bob....the possibility I referred to earlier remains? I suppose it just might be the case that the thumbnail browsing behaviour was changed quite intentionally for some reason, and is now working as expected? If so, then I hope I've managed to make clear by (yet another) example, why I consider the change to be a major step backwards.