Claude Peyriguère, of the French publisher Editions Hatier, to whom I mentioned the issue, gave the following details:
“The st and ct ligatures are indeed characteristic of the Pléiade books. They are related to the choice of the Garamond font. The features of this font partly derive from its former use, which rendered the style of handwriting. Unicode provides Latin ligatures (FB00 to FB06) as well as the st ligature. But, curiously enough, the ct ligature is missing.”
More information in Wikipedia.