The BadTitle exception is nowhere raised in the library. Either we mark it as deprecated or we start using it again and raise it to be compatible with compat. Maybe InvalidTitle subclasses it, as the Link constructor is afaik the only place where we would encounter it now. There is T106954 which raises InvalidTitle on the result from the API using a bad title. On other queries it doesn't specifically says that a title like Talk:Project:Foo is invalid.