Kievan Rus' is medieval state, it was a big part of medieval Europe territory as well as part of medieval historical process. 

There is strange region called "Rutenia" in place of Kievan Rus' in game. For some reasons name is wrong and it has Russian coat of arms and Russian culture what is not correct as well because Russia formed much more later that Kievan Rus.

Since Crusader King 3 tend to be historical game it would logically to fix this issue and add Kievan Rus' to the game with correct name, correct  coat of arms and culture.

If Paradox Interactive historical experts are not familiar with medieval ester Europe it is not a big problem, even in Wikipedia a lot of information about Kievan Rus'

https://en.wikipedia.org/wiki/Kievan_Rus%27