SharePoint Server 2019 will have no new features. Any new features, such as a new web part that is in-box, will only be available on vNext.
With that said, the Group calendar web part looks at a Microsoft 365 Group calendar. Microsoft 365 Groups are a cloud-only service and not available on-premises so it is very unlikely the Group calendar web part will ever come to on-premises unless there are new hybrid features, which again would only be in a future version of SharePoint Server.