Собственно, сабж: https://www.phoronix.com/news/KWin-Wayland-X11-Split
Yesterday marked the milestone of KWin's kwin_x11 and kwin_wayland code being split up. The Wayland and X11 code for the KWin compositor is now separate from each other but can be co-installable for systems wanting to support both X11 and Wayland environments.
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KWin_Wayland is being treated as the de-facto «kwin». KWin developers are planning to continue supporting the KWin X11 code until the Plasma 7 major release. However, no new features are expected to be back-ported to the KWin X11 code and in effect the X11 code under KDE was already treated as an informal feature freeze.