>but the character of his output had changed to the extent that it was not published in technical journals as it had been

Grothendieck didn't publish much in journals at all, ever. He was famous for writing books, not technical articles, such as the EGA and SGA series. His later work was the same, and the resulting manuscripts were widely sought because they were really good stuff.

Before he died he also claimed to have many tens of thousands of pages more that would never see the light of day, and people have sought those documents ever since.

I'm quite familiar with his output because my PhD is in algebraic geometry, and I've worked through his (known) writings extensively.

Here [1] is his complete list of publications. You'll notice by far his output is books and lecture notes, with very few papers at any point in his career.

[1] https://webusers.imj-prg.fr/~leila.schneps/grothendieckcircl...