Functioning labels are illogical.
How any person functions is contextual. Are you a good parent? A good reader? How good are you as a skateboarder? As a white-water rafter? What if you couldn't have any coffee this morning, how would you be then? What if you have the flu? So, how anyone functions is based on context.
Functioning labels are depersonalizing, dismissive.
Yes, there are people needing 24/7 care. Stephen Hawking is one of them. But, one would never call him "low functioning," as he can hear you, and he'd likely use his communication device to make you acutely aware of his personhood. Even when Autistics can't speak, we can hear, think, and communicate (in some way) our response to someone hanging such a dismissive label on us.
Functioning labels either limit expectations ("low functioning") or set one up for missing out on needed supports ("high functioning.").
Functioning labels are ableist. Autistics didn't cone up with a label designed to indicate how easily someone can or cannot pass for neurotypical. Autism is a beautiful neurotype in it's own right. We are not broken neurotypicals, we are whole Autistics.
Functioning labels originated in the same dark ages which gave us both the damaging normalizing "therapies" (often resulting in internalized ableism and C-PTSD,) and the outrageously presumptive term "mental age."
What term can we use, if not functioning labels?
Try "Has high support needs." (....in general, or in a specific context.)
We need to ditch the stigma of needing supports.
Currently, the ASD Levels 1, 2, and 3 are used to give a hint of where someone is on the spectrum.
Autism is not a linear spectrum anyway, more of a 3-D spectrum, or a sundae bar.
In North America, a Supports Intensity Scale is often filled out by the Department of Developmental Services, even before someone gets help from an autism agency. Supports Intensity Scale avoids broad-brush functioning labels, and customizes degree of supports.
Customizing supports is critical because functioning labels miss the mark.
Degree of autism severity actually has nothing to do with "functioning level."
You can have someone with no speech, poor self-care skills, needing much academic support ("low functioning") who only has mild autism-- rather flexible thinking, milder sensory sensitivities, and is more comfortable relating to people.........
And you can have someone with excellent speech, good self-care skills, academically needing few supports ("high functioning"), who has severe autism-- very rigid, inflexible thinking, significant social challenges, strong sensory sensitivities, frequently drifting attention, who melts down over every little snag.
Socially, within the autism community, there is no better-than-you hierarchy of diagnosis. Most today know IQ scores for autistics aren't likely to be reliable or valid, due to the nature of the testing, and our challenges with communication--both expressive as well as receptive, delayed processing, challenges with attention/focus, etc. Unstandardized minds often don't shine in standardized tests. While some autistics have a degree of intellectual impairment, standard intelligence is a poor measure of worth of a person.
Personally, I think it's time to retire functioning labels.