Yep. The most jarring changes of style, worldbuilding, themes and characters are in the first four books (Pratchett once responded to a question about whether "the Patrician" in The Colour of Magic was the same man as Vetinari by saying yes, but he wasn't the same writer). The non Alzheimer's explanation for changes in vocabulary around the turn of the century is that he'd started writing Young Adult Discworld books (he'd written for younger audiences before, but in very different worlds) and for better or worse some of the changes bled through to his other works. Of course, that's also compatible with him having less capability to write complex books, but then I'm not really convinced Thud is less complex than The Colour of Magic...
They do acknowledge that in the notes, referencing one other study (n=3) with one healthy aged author that did not exhibit these signs.
>Relatedly, as this is a single-case study, without a control author, it remains possible that the decline in lexical diversity reflects natural ageing. But note that previous research [15] has found that healthy authors can maintain stable linguistic diversity into their late 80s, suggesting that the decline observed in Pratchett’s TTR is indicative of pathology rather than typical ageing.
IMO still all very inconclusive but an interesting avenue to explore.
True. I've noticed this in authors like M.A.R.Barker and Ursula LeGuin, their later works when they were older were very, very different from their earlier ones (in Barker's case the 20-year gap between the two groups probably made it a lot more obvious). If I'd encountered them as being from some random unknown author I probably wouldn't have read them.