I'm asking which Gmail APIs would Thunderbird have to keep up with. As far as I know, it never used that API, hence it doesn't have to keep up with it.
Basically, the only thing that is kind-of Gmail specific is the auth support (https://bugzilla.mozilla.org/show_bug.cgi?id=849540). If Gmail sometimes in future switches from OAuth2 to some other protocol, the client will have to be updated.