What practical dysfunctionality characterizes severe autism? I have only known one autistic person and as far as I could see he was perfectly functional except that he could not relate to other people. It seemed as though most of the stuff his parents and others were trying to get him to do were not things that he actually needed to do, but were various forms of human ceremony, like looking people in the eyes and other things like that which have no practical necessity, but are just societal norms.
The main practical failure I saw in him was an inability to convey complex plans or ideas to other people.
So, I am curious what genuine functional disabilities severely autistic people have, as opposed to social misbehaviors. As far as I could tell, by reading complaints of parents of autistic children, the only true dysfunctionality I could detect were the following:

Ignoring traffic, walking into moving traffic without looking
Failure to discriminate between potentially dangerous strangers and friends
Failure to recognize dangerous objects like knives
Increased propensity to enter bodies of water and drown

Now, of course, all children suffer from the above dysfunctions, and it appears that autistic children simply are more likely to endanger themselves. Does this tendency to be unaware of these dangers continue into adulthood, or do adults with autism learn to respect water and other dangers the same way non-autistic adults do?
Autism is an extremely broad and multidimensional set of difficulties. 2 severely autistic person does not necessarily have the exactly same set of difficulties. There are various psychometric tests and questionnaries (Related link: How do we measure autism severity) for measuring severety of autism, but you may call it a total or overall measurement of difficulty. The exact set of difficulties vary from one autistic individual to another.
If you’ve met one person with autism, you’ve met one person with autism.
If 2 person shows symptoms of autism severely that does not mean the 2 person will show the each symptom with same severety. Their areas of strengths, deficits, sensitivities etc. can be entirely different.
The functioning labels  indicate how much support an autistic person needs, with level 1 meaning relatively little support is required and level 3 indicating the need for a great deal of support.
Update:

So, I am curious what genuine functional disabilities severely
autistic people have, as opposed to social misbehaviors

I am afraid why social dysfunctions are &quot;not genuine&quot; one. The social misbehaviours come from some cognitive problems, such as context blindness, prosopagnosia, etc.


Ignoring traffic, walking into moving traffic without looking Failure
to discriminate between potentially dangerous strangers and friends
Failure to recognize dangerous objects like knives Increased
propensity to enter bodies of water and drown


Sounds like severe intellectual disability, an often overlapping condition with autism. Severe autism certainly means severe impairment in some aspects of cognition, but not necessarily all aspects of cognition and intellect, and may have advance or even gigantic ( Autistic savants ) intellect in specific areas of cognition.

The main practical failure I saw in him was an inability to convey
complex plans or ideas to other people.

Similar other failures the autistics may experience, are prosopagnosia, dyscalculia, dyslexia, dyspraxia, issues with sensory processing (including meltdowns and shutdowns), problems with processing multiple sensory inputs, working memory problems, and many other. Autistics also may face anxiety and confusion with unpredictability or sudden changes in a plan or routine, These problems are very practical, serious and debilitating.