The Pakistani nation
That concept has not gelled yet--and periods of military dictatorship don't help. Just travel out of Karachi a bit and you'd find such writings rather frequently. This one was pictured in Gharo. Still, I gather that among Sindhis the vow for separation is not a prevailing sentiment, it is a loud cry of a very determined minority. For, compared to the East Pakistani society the Sindhi one has a strong feudal structure, and the feudal looks for his own personal interest first. There is a strong separation movement in Balochistan too, but that province of Pakistan is even more feudally oriented than Sindh. And in this age freedom struggles need popular support for fruition.
[For those who can't read Sindhi/Urdu, the sign says, 'Sindhu Desh, Zindabad.']