OpenGL GLSL‘s gl_ClipDistance is equivalent to [[clip_distance]] attribute in the Metal Shading Language. However, OpenGL supports disabling/enabling individual gl_ClipDistance[i] on the API level side. Writing to gl_ClipDistance[i] in shader will be ignored if it is disabled. Metal doesn’t have any equivalent API to disable/enable the writing, though writing to a clip_distance variable automatically enables it.
To emulate this enabling/disabling API, the Metal back-end uses a similar implementation as what Vulkan back-end does. Please refer to that document for more details.