If you are still on the beta programme that seems to slow the arrival of OTA updates. As long as you are now on a non-beta Android 9 you can sign out of the beta programme without any hassle (if for any reason you are still running a beta version then signing out will get you an update which will reset the phone, but unless you've received no update since August that should not be the case).
Why they don't automatically de-register people from the beta programme when it ends I don't know. They send you the non-beta update, and the beta programme doesn't roll over to the next major release (so if you want to try the 9.1 or 10.0 beta you have to sign up again anyway), so there's no point in no deregistering everyone when it ends, but they don't do it. And many people have reported only being able to get the October update after de-registering...