Would Sir Terry have appreciated or approved of this?

(Though I guess that cuts both ways and you could argue that writing more simply using the established parts of your legendarium is exactly what you might aim for if you were less consistently able to handle complex plot threads and novel worldbuilding)

The diagnosis was announced in 2007, meaning the shift occurred in 1997. 1997 was after Jingo and before Carpe Jugulum and The Last Continent, and 2007 was after Making Money and before Unseen Academicals.

The Last Continent is the first identified in the paper as below the cutoff for adjectives which they use to identify the start of the decline.

My own feeling is that many of his strongest works were before 2000, though he had several excellent ones after (the City Watch and first two Moist von Lipwig; I know the ongoing Tiffany Aching series are good, but in terms of writing I found them not as intricate as his earlier books.) I found Snuff harder to read, and Raising Steam, sadly, very difficult. I could tell the genius was there, but my memory of the writing was that it used much longer sentences, had less intricate plotting, and far fewer puns and wordplay. It was this book that made me really feel a sense of grief for what was happening to him, and it was this one where I first felt there was an observable threshold that was crossed.

I have sometimes wondered if it would be respectful if another author was brought into assist in editing or rewriting his last two novels. I know his unpublished works were destroyed, and any writing assistance is not his own voice. Yet I feel, in a sense, seeing books with such clear decline could in itself have let his legacy down. I don't know what his own view was or would be. While I admire Sanderson' continuance of the Wheel of Time, I would not wish such a drastic change in tone for some similar effort for the last of Pratchett's works. Yet I deeply wish that his last books were, somehow, different, more representative of him that I feel they were, in that his illness (in a sense, of course!) let him down. They cause me sadness.

GNU Terry Pratchett. (My own site sends this too.)

Did they compare to authors with long careers who did not develop dementia?

Maybe "decreased lexical diversity" is simply natural artistic progression, and not a bad thing.