It depends on which titles you own. They typically remain in print for half a century and more (they start contemplating a new edition after 40 years in print, normally), and most titles can remain available over a decade after their last printing.

If it can reassure you, they like to quote the collection manager saying "s'il est difficile d'entrer dans la Pléiade, il est encore plus rare d'en sortir."

Abandoning titles is definitely the exception rather than the norm. It's happened to more minor authors that they probably should have waited longer before they included them in the first place and that have totally fallen out of fashion and don't sell.

They've totally abandoned L'Encyclopédie collection as it proved no longer viable financially, but most of the works have resurfaced in Folio.

The collection is a huge challenge for them, though succesful enough to be worth it. Storage to ensure these books remain in pristine condition sometimes for years is very expensive. They also have their own dedicated printer and binder and there's a limited number of books he can make each year as they're partly handmade. With 566 titles published, it's become a challenge to keep them all available, but most still are.

They're not about to abandon it. On the contrary, Gallimard is convinced if physical books are to have any long term and widespread future it will be with such upper scale editions (French publishing is currently in serious denial about ebooks). The Pléiade edition is also a major source of scholarship Gallimard recycles/adapts for his Quatro and Folio collections.

I know for sure you do own a few titles which are OOP, though. The medieval chroniclers, for example. That's from their old medieval collection. Renart, CdT etc. are from their new one.