The Optimism chain has been renamed to OP Mainnet, aiming to distinguish its relationship with other Layer 2s such as Base

ChainCatcher message, Optimism officially announced that the Optimism blockchain has now been renamed to OP Mainnet. This change was made to distinguish the OP Mainnet blockchain itself from the community, atmosphere, spirit, and so on that Optimism encompasses. OP Mainnet is just one of the foundational layers of many Layer 2 "superchain" networks, seamlessly communicating with other chains like Base, Zora Network, etc. (source link)