For right now OpenGL over GLX is the default, but as of the latest development activity, OpenGL over EGL is also now very possible. The back-end can be controlled by the KWIN_OPENGL_WS environment variable.
The deprecation of GLX in favor of EGL should happen with the new Linux OpenGL ABI. EGL is also supported by Wayland with no support for GLX.
More information on the OpenGL EGL support in KDE's KWin can be found via Martin's blog.