Akanje is a great way of distinguishing between the Eastern Slavic languages. Ukrainian doesn’t have it, Russian does, but only in speech, not in written form, and Belorusian has it in written form as well.
Northern dialects of Russia didn’t have akanje. Only southern dialects. Southern Russian dialects were not spoken in geographic south of Russia. They were spoken just south of Moscow including Moscow itself towards Belarus. Because spoken language of Russia became similar to what people of Moscow speak , so akanje is found all across Siberia and major centres of Russia. As you stated Russian has only akanje in spoken form. There are also some differences between Belarusian and Russian akanje that only linguists can explain.