It's just the normal "Pager" widget, configured to show application icons.

I find "minimap" more descriptive for what I'm doing, because I don't minimize, nor stack windows, so if a window exists, it has a location.
Which is also ultimately how I use this thing. Imagine a large desk where you need to jump between topics every so often. You'd put related sheets of paper next to each other and leave a bit of space between the groups. Sheets of paper are just application windows in my case (I will open one or more windows per task, I don't mix tasks together based on application like people usually do). Well, and my desk also happens to be very long, so I can comfortably fit a minimap for it in my panel.

And because I really like multitasking, I've actually got multiple desks, in different colors:

For these, I use Plasma's Activities. The different colors are done by having a transparent panel and then setting the wallpaper to different colors + telling Plasma to use the wallpaper for determining the accent color.

In this screenshot, you can also beautifully see a workspace with 5 Kate windows, which is genuinely where I shoved a bunch of notes, for me to sort through them later. 🙃