Is "Pledge to Protect" available in the original?
Yes, you can pledge to protect City-States without the Gods & Kings expansion. The mechanic changed for the expansion however, now allowing you to pledge to protect city states immediately, while before it required you to at least be friends with the city state before you are allowed to. For some reason, the AI is still allowed to pledge city states right away, even in the base game. G&K just evened the playing field in this sense, but you won't need to buy the expansion to use the mechanic.