He is still actively publishing! TAOCP Vol 4 Fascicle 5 (which is going to be the first one-third of Volume 4B) came out less than a month ago, and it contains

He is still actively publishing! TAOCP Vol 4 Fascicle 5 (which is going to be the first one-third of Volume 4B) came out less than a month ago, and it contains a lot of material and ideas that have never been published anywhere before. To answer your specific question, I'm positive he's referring to Dancing Links: see section 7.2.2.1 in the published fascicle, or his draft (pre-fascicle 5c) at https://cs.stanford.edu/~knuth/fasc5c.ps.gz https://cs.stanford.edu/~knuth/fasc5c.ps.gz -- the idea originally came from Hirosi Hitotumatu and Kohei Noshita (1979), then Knuth popularized it under the name "Dancing Links" (2000?), then the idea of "Dancing with ZDDs" was by Masaaki Nishino, Norihito Yasuda, Shin-ichi Minato, Masaaki Nagata (2017), and he's extended it to Algorithm Z in this publication.