The intention of the Unicode Regional Indicator Symbols was to represent current countries in existence (as encoded in ISO 3166-1 alpha-2), as locales for software; the flags are a lagniappe. (A rather unfortunate lagniappe.)
Hence, no SPQR flag: the point of the codes is to indicate the country you live in, as a two letter ISO code, to be used for software localisation. (E.g. use Serbian rather than Russian italics for Cyrillic.) The Roman Empire doesn’t have such a code, because you don’t live in it—though a bunch of defunct 20th century states do.
There’s a proposal to throw the door open for Emoji flags to regions such as Scotland and California. Per UTS #51: Unicode Emoji, those are still restricted to present-day national subdivisions, and historical states are not in scope for them.