I have no clue where to put this since thumbnail-grid has been merged into kdeplasma-addons and I can't seem to find any issue trackers that are open for this sort of thing so, screw it, forums. It's a really minor usability gripe but in most windows where things are displayed there is left click to directly interact, (most often to switch to that thing that you're clicking on) right click to get more options, and middle click to either quickly close the window/tab or quickly open an additional window/tab and put the thing that you clicked on in it. Unfortunately the thumbnail-grid task switcher doesn't follow this functionality from what I can tell, as I can middle click on windows and nothing will happen.

IMO any task switcher should allow the user to quickly and easily terminate programs with middle click. There is an X button on each item displayed within the task switcher, but if you want to close lots of windows at once that can be much more tedious. Middle click to quickly close is functionality that has been more or less completely integrated into the rest of KDE (at least as far as I can tell) but not having it be activated for the task switcher makes it inconvenient to quickly close an arbitrary set of windows.