Microsoft quietly announced yesterday that Slow ring members of the Windows Insider Program will be updated to Windows 10 20H1 "very shortly" unless they switch to the Release Preview ring by updating their settings before the update hits.
Yesterday's release made 19H2 Build 18363.418 available to Release Preview members of the Windows Insider Program using Build 18362.10024. Slow ring members who are still using that build--they were previously given the option to use a build from 20H1--were advised that "Now’s the time to double-check your Windows Insider Program settings!" if they don't want to leave the 19H2 development branch.
As the name suggests, Windows 10 20H1 is expected to debut in the first half of 2020. Some of the Windows Insider Program's members in the Slow ring are already on that branch, but others are using Windows 10 19H2, which is slated for release in November. People might want to know they're about to switch to a new development branch if they don't switch to the Release Preview ring soon.
Yet Thurrott noted that Windows Insider Program members could have easily missed this announcement because Microsoft updated an old blog post rather than publishing a new one. Updated posts don't appear in RSS feeds. The only ways to find out about this change would've been to randomly check the blog post or follow the Windows Insider Program on Twitter. Most of the program's members don't.
Still, this doesn't come as much of a surprise. Microsoft pushed Skip Ahead members of the Windows Insider Program to the 20H1 development branch in February; Fast ring members joined them in April. With only a few weeks left until 19H2 reaches most Windows 10 (opens in new tab) users, it makes sense for Microsoft to finally push Slow ring members to 20H1. It's called the Slow ring, not the One Development Branch ring.