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I assume you're following this YDLiDAR manual or similar, but the turtlebot-rviz-launchers package is indeed outdated (its GitHub repo was last updated 6 years ago). The YDLiDAR GitHub documentation is a more up-to-date resource.

As a side note, RViz is installed with the desktop ROS installations, so you may already have it. If you don't have RViz from the ROS install and need it, Solrac3589's command is correct:

sudo apt-get install ros-melodic-rviz