The phrase "not supported" is a term of art meaning "may do something strange and undesirable under some conditions, may not perform consistently across versions, and should not be relied upon by code purported to be reliable." It need not lead to terrible consequences consistently, or ever depending upon the local meaning of "terrible". But when anybody uses unsupported features, they are choosing to forego any support for whatever results from such use. You might buy some rappelling line and hardware rated for 5000 Newtons of tension strength, and find that the first 10 articles test to 3 times that. If you then decide to use it to lower yourself in your car down a cliff, you might live to tell about it. Or, if the line parts partway down, in your final few seconds you can tell yourself, "I guess nothing more than 5000 Newtons is 'supported'."