The Release Preview Ring, which usually previews a full release by a week or so, saw 17763.1 at the same time as the Windows 10 October Update was announced yesterday.
Release Preview users were waiting to see the latest version before it was released to consumers, but it never arrived. This is a change in how the rings work, but it was done on purpose according to Brandon LeBlanc on the Windows Insider Team.
Release Preview is now for servicing validation and rollout testing.
No. Technically we released it to the Release Preview *at the same time*. And we did this on purpose. We're using RP for servicing validation and roll-out testing which is why we held it for today.
— Brandon LeBlanc (@brandonleblanc) October 2, 2018
This is a stable release and yours truly has tested the install on Release Preview.
Don’t Forget to submit feedback in the Feedback Hub App on Windows 10.
You can also find more information in the official blog post.