Most discussion of citizenship in the empire focuses on the republic or early empire. So far as I can tell, "popular" histories pretty much drop the subject after Caracalla's expansion of citizenship.
Basically, I'm wondering what happened afterwards, especially a century or two down the line as we get into the late antiquity. Was the concept of citizenship still an important one? Was it still the near-universal version that Caracalla mandated, or had it reverted to the more limited concept seen in earlier times?
Also, do you know what some of the later legal reforms had to say on the subject? Things like Justinian's Corpus Juris Civilis or Basil's Basilika?
I'm actually bad with the legal aspects of Roman citizenship, even late Roman. /u/peripatos might be more helpful as he specializes in Roman law.
I can tell you generally that being a Roman citizen survived in the early parts of the early medieval era in the form of either still being liable for taxation under the remaining "Byzantine" empire, or in the form of having the right to be tried under Roman law under particular barbarian kingdoms.
But as taxation and legal infrastructure waned, being "Roman" in those places was considered less a general identity, and more an elite one amongst particular segments of the aristocratic population that valued the continuation of such identities in opposition to the main ruling barbarian class, like the romano-gallic "senators" of southern gaul.
By the 700s, almost no one referred to themselves as Roman anywhere in western europe, except maybe in the city of Rome itself, and of course by those who were seeking to lay claim to continuity with the imperial power of old, i.e. Charlemagne and the other Holy Roman emperors.
After flicking through the Justinian Code I am definitely of the opinion that in Late Antiquity after Caracalla's edict in 212, manumission and citizenship were still important issues that emperors continued to take involvement in. Let's take a look at one pertinent example:
"Emperor Justianian to Johannes, Pratorian Prefect: We therefore ordain that if anyone wants to give his slave freedom by letter, he may do so in the presence of five witnesses who can verify the document by adding their signatures thereto below the writing of the autor thereof, whether such author has merely signed it or whether it is wholly written by him. And if he does so through a letter, written by himself or by a notary (tabularius), freedom accrues to the slave, as if he received it through a codicil, and he shall be free and a Roman citizen even during the lifetime of the patron himself" (CJ, 7.6.1).
There were others as well, mostly concerning situations like slaves who were owned by multiple parties and manumitted and given citizenship (which was a tricky situation, since a person couldn't be half free and half a slave) (CJ 7.7.1). But by far the most important edict concerning citizenship that occurred post Caracalla was under Justianian. He did away with the various degrees of citizenship (such as dediticii and Latins) and created a single blanket citizenship (as dealt with in CJ 7.5-7.6. Which is a lot of text, so I won't shove it all into this single post). So after Caracalla there were still reforms being done and the idea of Roman citizenship wasn't quite universal.
For further reading, I would recommend Sherwin-White's "The Roman Citizenship" (Oxford, 1939, revised 1973) and Fred Blume's "Annotated Justinian Code".
In fact you can find the latter here