What modern form of government is closely linked to Voltaire's?
Voltaire lived from 1694 through 1778, and thus lived entirely under the "ancien régime", the monarchy established in the Middle Ages that was not overthrown until the French Revolution of 1789, which Voltaire did not live to see.
The ancien régime was an absolute monarchy, with few restraints on the power of the hereditary king. Louis XIV famously claimed "L'etat c'est moi" ("the state, it is me"). The people living in France were divided into three estates, the clergy, the nobility, and the commoners, and laws and prerogatives were dependent on membership in one of those estates, with many positions of power reserved exclusively for those with noble blood, and commoners having few rights or liberties.
Absolute monarchy is not really a modern form of government, although a few countries such as Brunei, Oman, Qatar, Saudi Arabia, and Swaziland still retain it.