In the original whitepaper Satoshi proposes a creation of a timestamp server to authenticate that Bitcoin blocks were not created later than at a given time. Has this feature been ever implemented or planned to be implemented? Why would the timestamp server be useful to the Bitcoin client/protocol/community?
The system called timestamp server in the original whitepaper is currently more often referred to as a "miner".
They timestamp blocks, which combine several transactions, imposing an authorative order on these transactions. This is necessary as the system cannot otherwise decide which of two conflicting but otherwise valid transactions should be accepted.