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 features, 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.
Information about products not manufactured by Apple, or independent websites not controlled or tested by Apple, is provided without recommendation or endorsement. Apple assumes no responsibility with regard to the selection, performance, or use of third-party websites or products. Apple makes no representations regarding third-party website accuracy or reliability. Risks are inherent in the use of the Internet. Contact the vendor for additional information. Other company and product names may be trademarks of their respective owners.