I can maximize windows under X11 dragging them to top of my screen using my touch screen, but I can't get the same result under Wayland.
I'm not able to reproduce. Moving the window with touch to the screenedges works as expected. Please note that pixel 0 triggers, so you might have to overdrag a little bit and your finger needs to touch that pixel.
weird. I have this problem on both Arch (plasma 5.10.3) and neon dev.
you could check in KWin debug console whether you get the touch events at the border at all. Maybe the touchscreen is miscalibrated...