Support for GTK apps has been improved as well in KDE Plasma 5.21.3 by align the height of the menus in all GTK apps with those in KDE and Qt apps, as well as to force GTK apps that rely on the new Libhandy library to display their top headerbars with the right height.
https://9to5linux.com/kde-plasma-5-21-3-is-out-to-improve-the-new-plasma-system-monitor-app-and-support-for-gtk-apps
https://developer.puri.sm/Librem5/Apps/Guides/Design/Adaptive_UI/index.html
People should not use gtk3-nocsd or gtk3-classic from now on. In other words: future is with CSD and DWDs