Microsoft has announced changes made to their Windows Insider program. The Skip Ahead ring, used to deliver early development builds, which were ahead of the upcoming feature update, is now closed and not available for joining.
Microsoft has explained that they are interesting in shipping most recent pre-release Windows versions to the Fast ring instead.
Starting with Build 19018, the Skip Ahead option won't appear on the Settings > Update & Security > Windows Insider Program.
Insiders may refer to the Flight Hub web site to check out what build in which ring is currently available.
If your PC has been configured to receive updates from the Skip Ahead ring, it will be automatically switched to the Fast Ring.