It's far too easy and tempting to join Google's Android beta program. A click of a button to enroll your device and an OTA update later you're helping Google test its latest and greatest software before the public.
Leaving the program, however, isn't as seamless. Indeed, you still click on a button and install an update, but at the end of that process your Android device has been factory-reset and everything on it is gone.
There is, however, a way to get out of the beta program and not have to set up your phone, tablet or watch from scratch. Unfortunately, it's not very clear on the beta program website. In fact, I recently left the program with a hope and a prayer I wouldn't have to factory-reset my phone in the process.
The key is to leave the beta program when you have a current official version of Android installed on your device, not in the middle of a beta.
For example, Google launched the Android Oreo 8.1 beta program awhile back, and I promptly enrolled a Pixel 2 XL in the program. A couple of weeks ago, Google made Android 8.1 official and released it to the public. Once the official build of Android 8.1 was installed on my Pixel 2 XL, I unenrolled from the program. Instead of my device showing a pending update, there were no further steps required on my part.
My device is running the same exact build as those who didn't participate in the beta program, thus no device reset is needed. If I had left the program any time prior to installing the official, public build of 8.1, I would have been forced to factory reset my phone.
The easiest way to compare the build number you're currently on with the official version is to compare the build number found in Settings > System > About phone > Build number on your device to the official build number on this webpage. If the build number for your device matches the factory image posted by Google, then you're good to leave the beta program.