This is a renaming. They were renamed since the 'provisional' GenesisMethod of generating the system channel for orderers was removed, and we now expect users to use the method of generating a genesis block using configtxgen to bootstrap the orderer. Though I believe we still support either, the first set is deprecated and users should use the new names to prevent surprises in the future when the legacy names are removed.

 

You can find information, including the associated Jira numbers in the 2.0.0 release notes: https://github.com/hyperledger/fabric/releases/tag/v2.0.0