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It just simply is 'ROS 2'.

The developers of ROS 2 have named it ROS 2, as evidenced by its usage on its homepage https://www.ros.org/ and throughout all documentation. The name of a command line tool that is only a small part of the full ROS 2 ecosystem does not change that. Similarly, you use python3 or maybe python3.10 rather than python\ 3 or python\ 3.10 to work with Python 3 or Python 3.10, and you don't write those Python3 or Python3.10.

Having said that, everybody will get it when you use ROS2, or Ros2 even, like anybody will understand when you use 'Coca Cola' or 'Micro Soft' (though perhaps with judgment :) ), and anybody making a big point of it, in any case in conversation and out side of official documentation, can be safely ignored IMHO.