If Russian miners can not see blocks coming from the rest of the world and the world does not see Russia's, then does it not lead to forking?
I believe o_e_l_e_o meant something else. If you have 75% of the hashrate and I have 25% and suddenly we lose connection, the blocks continue on my side as if you disconnected and on your side as if I did. You still have a significantly higher hashrate than I do, which makes it easier to solve blocks for you than it is for me. Since to me it is way more difficult to solve blocks, by the time difficulty changes you will have mined more blocks than I did. The difficulty changes, so now your chain continues to be first although I do not see it yet. Now the question remains, do we continue sideways until we connect and my progress vanishes in front of your chain or do we fork? The safest and most logical way is the latter, unless the Russians want to play a Bitcoin Russian roulette.
It doesn't have multiple chains, it has one. Which means if there is a blockchain the whole world agrees to, and we all use it, then suddenly Russia goes out, just Russia goes out, the blockchain continues like nothing happened. If Russians are somehow trying to connect to it, they will fail, so they will have to create their own coin.
Consider it like BCH, bitcoin cash was created from Bitcoin blocks as well and became its own coin, Russians basically will have that. Bitcoin has only one, and it can't be separated like that. If you really want to find a better way, they could just use a VPN to access bitcoin even if the government blocks them.