The Relational Database Service offers now also Microsoft SQL Server 2019 Enterprise / Standard Edition.
Currently the versions Microsoft SQL Server 2016 & 2017 Enterprise / Standard Edition, as well as the 2014 Standard Edition are available.
Please kindly visit Microsoft Release Notes for Microsoft SQL Server Version 2019 Enterprise / Standard Edition to get more information about new functionalities on this version.
Further information can be found in the RDS area of the Help Center.