My guess is they detected being scraped and did this as preventive measure.

My guess is they're cozier with publishers now than 20 years ago when they fought all the way to SCOTUS.

"Hey, remove search?"

"OK, it was costing money anyways."

my guess is that the copyright landscape changed due to AI training, and these publishers won't let Google use that data anymore

The books are still there, it seems like the rankings have changed though.

If search gives you a preview with a few surrounding words, it is fairly simple to abuse search with quotation marks to extract bigger and bigger sections of the books, potentially till you have the whole book.