Violating the API Terms of Service [1]?
> [...] Further, you will not: [...] (C) access our APIs or documentation in order to replicate or compete with the Services;
> [...] Further, you will not: [...] (C) access our APIs or documentation in order to replicate or compete with the Services;
I’m definitely not a lawyer, but it’s not clear to me that this would violate the ToS.
They can certainly attempt to block 3rd party clients, though. Or threaten to / actually sue, even if they’re unlikely to be successful.
Regardless of the upthread "well, just switch [your company/government/entire friend network] to an open/permissive service" non-answer, I think there's going to be ever-more need for unofficial clients for some of these services, and a big part of that is going to be how to avoid having them shut down immediately by service providers...