His website [1] claims he is retired. He goes on to saying how he is like a hermit and needs long batches on un-interrupted time. Worth a look, he's an interesting fellow.
Emeritus is basically retired. You have continued access to university resources, but little/nothing is required of you. Source: my wife happens to be a professor at the same university.
> In 1968 Knuth joined the faculty at Stanford University, and served until his retirement in 1993, after which he was designated Professor Emeritus of the Art of Computer Programming. [1]
What's the difference between being an active-duty professor and an emeritus professor? It means he didn't have to teach classes, do much (any?) departmental service, or be in active communication with administrators. All of these things require email these days, which is why other professors can't "pull a Knuth".
So while folks like to trot out the story of Donald Knuth, we should keep in mind the context. He's not a regular professor, or even a regular senior professor. He is — and has been since 1993 — an emeritus professor who has practically no day-to-day responsibilities at the University.
1: https://www.encyclopedia.com/people/science-and-technology/c...