During the ETH/ETC fork replay attacks were terrible until people figured out how to make a contract that was guaranteed to act differently in the two chains; y

During the ETH/ETC fork replay attacks were terrible until people figured out how to make a contract that was guaranteed to act differently in the two chains; you would send your ETH to that address, providing two output addresses, and your ETH would go to one and your ETC would go to the other. Before that was commonplace there were numerous problems of transactions being replayed causing all sorts of problems. If someone wanted to fork post-PoS ETH, chances are they would have to do so very deliberately, with code on the new fork deliberately designed to prevent replay attacks, as was done in the BTC/BCH fork.