Earlier this week, Microsoft once again opened up Skip Ahead enrollment for those that want to test out Windows 10 19H2 before 19H1 testing is complete. But if you haven't signed up by now, you're out of luck, as it's now closed.
As has always been the case, the Skip Ahead subset of the Fast ring has limited capacity, so once a certain number of users enroll, no one else can. However, if those Insiders switch their machine back to the standard Fast ring, or switch it back to production, then space opens up.
Of course, once Windows 10 version 1903 ships, 19H2 builds will start coming to regular Fast ring Insiders. That should happen in late March or early April, so even if you missed your chance for Skip Ahead, you won't have to wait too long.