if ND is becoming more common than NT, then surely NT is no longer NT? Or, is it that us ND birds of a feather flock together, as it were?
Becoming more common doesn't mean majority, either literally or socially.
Neurotypical, like neurodiverse, comes from an autistic sociologist - typical is being used in a social way of what our current social structures treat as typical, what society is typically designed to accommodate. Even if we somehow became the majority, it wouldn't change that unless society also changed. It's like how women are a literal majority, but society in many ways treats men as the 'typical'.
Some conditions do become mostly 'typical' once we're well accommodated for. Most eye issues that lead to needing glasses are treat as 'typical', particular for adults - there can be a few issues, but society has adjusted to them being common place. I don't think society is generally moving that way for neurodiverse people, if anything it feels more separating off.
Yes, data supports 'birds of a feather' - and also that birds not of a feather tend to push apart. Multiple studies have shown neurotypicals tend to prefer other neurotypicals and negatively judge autistic people within 7 or so seconds (most just slightly) even if just watching a video without being told anything about the people in the video. Similarly autistic people have been shown to clock and prefer other autistic people over neurotypicals in similar types of studies. It's how the double empathy problem comes about - both sides can see the kindness and empathy of themselves and struggle to see it in the other.
Where are these stats from please?
WHO, NHS, and other places that pp have linked have looked into the current research and data on it.
With Autism and ADHD combined, we're looking at ~5% with current data, as we now recognise there is a significant overlap (previously, a diagnosis of one excluded the other).
Stats that are 10-15%+ (I've seen as high as 20%) are usually including more conditions like dyslexia, dyspraxia, dyscalculia and other neurodevelopment conditions as well as in-utero exposure disorders like FASD that cause significant neurodevelopment changes. Some also include certain neurological disorders like Tourette's and Schizophrenia that show a link with neurodevelopment issues. The same frequency research puts FASD to about as common to twice as common as Autism, including that there is an overlap there as well, but it gets a lot less media exposure. Including all of those, the number will get bigger even with co-morbidity, but there are some disagreements over how much to expand neurodiverse - which originally was just discussing autistic people & now firmly is used to discuss autism and ADHD, with some including others.