Not trying to be a menace, but I just tried it out using xdg-portal-test-kde and the screenshot portal definitely works on KDE Plasma 5.27.10.
If you are experiencing issues with that, please create a bug report for xdg-desktop-portal-kde so it can be fixed.
Unfortunately, you on GNOME and KDE you don’t have
org.freedesktop.portal.Screenshotmethod in/org/freedesktop/portal/desktop.Not trying to be a menace, but I just tried it out using xdg-portal-test-kde and the screenshot portal definitely works on KDE Plasma 5.27.10. If you are experiencing issues with that, please create a bug report for xdg-desktop-portal-kde so it can be fixed.