I am pretty sure it has always been there and I think it is part of the genesis block (though it might be called Network ID in there because early on the two were conflated)? It is necessary to keep testnetworks and mainnet separate from each other, and then eventually (with the ETC split) to keep ETC separate from ETH.
I meant to say that I think you should just have a separate address prefix for every chain, Goerli should not be called “Ethereum”, it should just be called “Goerli”. The same as I don’t think Polygon should be called “Ethereum” or Binance Chain should be called “Ethereum”. When people are talking about Ethereum, they are almost certainly talking about Ethereum Mainnet. There is a whole spectrum of chains that are similar to Ethereum Mainnet ranging from the testnets, to ETC, to side chains, to other blockchains that run the EVM Trying to draw a clean line is difficult and I don’t think it adds value. Each chain should have a globally unique name (which they all already do), and we should use that globally unique name as the prefix without trying to group/categorize them.