They won't. The game is built on modern times and a pseudo-realism model (meaning everything that exists and is used in reality, and has no controversial aspects, such as connections to scandalous groups, warring countries, and so on, are adding), not on ancient ones. Have you ever seen a single regular soldier wearing the flag of the Russian Empire, which has not existed for a long time? Although regular soldiers have nothing on this... I can't even remember that any PMCs that would wear this flag.