Currently task indicators are displayed as semi-transparent squares and a thin line at the edge of the panel, the only distinction between focused and background application is the use of accent color. This causes numerous problems for people with poor visibility:
1. The use of color to differentiate the state between applications is not bad, unless it is the ONLY method. People who have problems distinguishing the use of colors at a glance, or who prefer to work without the use of color, are going to have a serious problem when working since it is impossible to discern the state of an application if you remove the color.
2. Apps that aren't focused keep the background semi-transparent, filling up the panel quickly if you have multiple apps open, drawing attention in the same way as the focused app, and overloading the panel. This makes it even more difficult to see at a glance which application has the focus and is drawing the eye too much.
3. The line that is drawn at the edge of the panel becomes practically invisible depending on what content you have on the screen, making it practically useless. It would need to have at least a couple of pixels of separation with the border so that it does not blur with the rest of the content and people who need glasses can use it without problem.
4. The current theme does not fit well with the current use of the floating panel. The floating panel has a separation with the edges of the screen, and the widgets also separate. This repeats the problem of point 3, since the widget indicator is also a colored bar attached to the edge, and aesthetically it is not pretty to use the use of colored boxes for applications.
5. Grouped tasks do not correctly transmit their state. The use of an icon in the middle of the icon is not always visible as it is confused with the icon, and only conveys that there are "more" apps of the same style open, without giving an approximate number. Minimum 3. And it could not be indicated with duplication of the icon either, since it would cause the same problem that it is difficult for people with vision problems to see it. At that point "Fancy Tasks" does a good job.
Those problems are what prevent me from using the default theme and I would prefer it to be fixed. I've currently had to create my own theme to get it to work:
FWIW #4 is unrelated and being fixed for Plasma 6, and #5 is a different issue entirely; please report that separately.
#1-3 seem valid, though.