Exchange 2019 was released end of October 2018, on 11. December 2018 Microsoft released the official Exchange 2019 preferred target architecture (PA) document.
Compared to the previous PA that is available for Exchange 2013 and 2016, the Exchange 2019 PA was updated to accommodate changes to Exchange 2019 such as planning considerations for
The main architecture areas such as Namespace or Database availability group and database designs follow the best practices as mentioned in previous PAs. However they also include additional useful advice for Exchange Server 2019 and all earlier versions do not support the use of the Cloud Witness feature, first introduced in Windows Server 2016 Failover Cluster.
If you plan to migrate to Exchange 2019, this information is a crucial read so you can provide the best on-premises user experience for your environment. You find the Exchange 2019 preferred architecture document here.
Check out my blog on whether you should upgrade to Exchange 2019 here.