Briefly put, we know that Prussia is East Germany, but he hasn’t been formally introduced or referred to as such. 

On the wiki page, they cite “Germany is playing an impossible game” as the strip where Prussia’s status as East Germany is confirmed. It pretty much is [it refers to German Unification], but again, it’s not explicit. Technically, it’s not confirmed then, as I’ll elaborate on later.  

I actually disagree with how this was done. No matter how obvious something is, I’m of the mindset that things shouldn’t be accepted as canon until it is explicitly stated as such. Most East Germany references have to do with German Unification or Prussia working for Russia, however, no labels are given…

Guide on what we know about Prussia’s extremely probable status as East Germany coming up tonight!