Microsoft announced System Center 2019 for Q1 2019 in System Center Roadmap
Microsoft has published a roadmap where System Center 2019 is announced for Q1 2019:http://download.microsoft.com/download/5/9/6/59602436-C9C5-48A1-B305-827654B9077B/System_Center_Roadmap_EN_US.pdf . This version will support Windows Server 2019.
In Long-Term Servicing Channel (LTSC) Microsoft will released at a lower frequency. The LTSC will continue to provide 5 years of mainstream support followed by 5 more years of extended support.
Microsoft has rolled out the second upgrade of its System Center management suite under its new semi-annual channel release cycle called System Center version 1807. Microsoft customers with Software Assurance have the options for the semi-annual channel releases to receive upgrades at a faster pace but are only supported for 18 months.