However this is an entirely different question. Whether or not an API is copyrightable, and whether you'd be in violation for recreating a proprietary API for the purpose of creating an (open-source) market alternative for their product, are entirely different questions to whether or not a specific private company has stopped serving specific customers due to use of software they do not authorize (which they are entirely in the right to do, legally)
However this is an entirely different question. Whether or not an API is copyrightable, and whether you'd be in violation for recreating a proprietary API for the purpose of creating an (open-source) market alternative for their product, are entirely different questions to whether or not a specific private company has stopped serving specific customers due to use of software they do not authorize (which they are entirely in the right to do, legally)