Its complicated.
The general law of gratuitous licenses (in the US) is that they are revocable at will; terms of the license do not overcome this. Whether the restrictions in a license like this are limitations on the permissions it grants (leaving it gratuitous) or consideration in exchange for permissions (making it a license contract) is not always an easy question.
Further, even if things for which the license has already been uses might be protected, the license text is an offer, and the offer can be withdrawn. This is important, because the OGL 1.0a isn’t sublicensable; every new user making a new work directly licenses all ancestor works under it, rather than sublicensing through the immediate parent.
But, even to the extent it is recovable, the doctrine of promissory estoppel may limit the effect on parties who have already reasonably relied on the promise of irrevocability.