https://www.bleepingcomputer.com/news/microsoft/windows-10-preview-build-18219-is-now-available-with-improvements/

Microsoft has released a new Insider build in the "19H1" branch of Windows 10. The "19H1" branch is the next big update coming after Redstone 5 and is projected to be finalized in the spring of 2019 according to Microsoft’s typical release schedule.

Today's release of Windows 10 Preview Build 18219 is only available to Insiders in the Skip Ahead Ring whose availability was officially closed as of yesterday. This means that unless you had previously selected the Skip Ahead build, you will not be able to get builds form the 19H1 branch until they reopen it.