A "complete" Rosetta API implementation supports all methods in the Data API and Construction API specification. That being said, there is no gatekeeper that prevents your implementation from being used until all methods are supported. If developers wish to use a partially completed implementation, there is nothing that prevents this.
Note on Mempool Methods
/mempool/* endpoints were added to provide a better UX for blockchains
with large mempools (ex: Bitcoin and Ethereum). Many newly launched blockchains
do not have support for monitoring the mempool because transactions are often
included in a block shortly after broadcast (either because of a higher TPS capacity
or less transaction activity). If this applies to the blockchain you are working on, it is ok
if the mempool method implementation is deferred until mempool monitoring is