Citizenship, then as now, entailed a specific group of privileges and responsibilities. One of which was some degree of say in how Rome was governed (optimo iure). As the Roman Republic originally expanded from the city-state of Rome, full citizenship (Cives Romani) was initially limited geographically to Rome itself instead of outlying Latin holdings.
Who got what level of Roman citizenship was used as an incentive model to recruit the best and brightest from outlying provinces, especially during Rome's early growth. Holding Roman citizenship also invested a personal stake in the welfare of Rome, since either you or your ancestor may have gone to considerable lengths to acquire it in the first place.
Nobles in Rome would have always been full citizens of Rome. The status of aristocrats in the provinces would changed as Rome changed; but typically anyone you consider a "Roman noble" had, as a historical family unit, acquired full citizenship by marriage or adoption or relocation and naturalisation of the senior branch to Rome.
When the Edict of Caracalla (Constitutio Antoniniana) expanded Roman citizenship to all free males in the Empire; no one lost their existing citizenship. Technically the expansion of citizenship was primarily an expansion of responsibilities (taxes, military service) than an expansion of rather diluted privileges. It was part of the barbarization of the Roman Empire.
If feudalism marked a shift of citizens into subjects, this was due a shift in how the relationship between rulers and ruled was interpreted. Being rich and powerful was useful in and of itself, regardless of the concept of citizenship.
Modern citizenship (from a Eurocentric perspective) arose from concepts of social contracts and popular sovereignty that preceded the American and French revolutions. If you were an aristocrat that successfully integrated into the new political paradigm, you would have had the highest level of citizenship due to your level of property ownership.
In so far as citizenship was "reintroduced", French nobles quickly became full citizens once the head chopping phase had passed - that is, after the Thermidorian Reaction of 1794. The aristocracy and their descendants (private equity) have kept a lower profile ever since.
(Roman section of this answer applicable to Revision 1 of the question.)