X11 is no longer included with Mac, but X11 server and client libraries are available from the XQuartz project.
Apple created the XQuartz project as a community effort to further develop and support X11 on Mac. The XQuartz project was originally based on the version of X11 included in Mac OS X v10.5. There have since been multiple releases of XQuartz with fixes, support for new eatures, and additional refinements to the X11 experience. Apple is a contributor to the XQuartz project and has worked to ensure that X11 works as expected with macOS and latest available versions of XQuartz.
X11 server and client libraries for macOS are available from the XQuartz project at www.xquartz.org. Download the latest version available.
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