What is the use of the updated op_success operation code?
For now (or even post Taproot activation in November) they have no use and shouldn't be used in any scripts. However as the excerpt above says, making multiple previously disabled opcodes OP_SUCCESS in the Taproot soft fork means new opcode functionality can be easily added in future soft forks post Taproot. Old clients that don't update for future soft forks will automatically treat scripts with these new opcodes as valid and hence they won't be forked off the network by new clients validating the use of these new opcodes.
Other upgradeability features in Taproot (BIP 341) are discussed here.