For the last few years, Google and its partners have leveraged Project Treble to get preview versions of Android on more smartphones. For Android 11, Pixels have been the only game in town until now. OnePlus is now offering downloads of Android 11 for the OnePlus 8 and 8 Pro, but don't expect something stable enough for your daily driver.
Android 11 features reorganized notifications, chat bubbles, stricter permission control, a new quick settings panel, and more. That's all based on the Pixel, but the OnePlus 8 and 9 Pro will presumably get the same goodies. OnePlus seems to use the terms "beta" and "developer preview" interchangeably here, and given how quickly this release follows the Pixel beta, the OnePlus builds may very well be based on one of the previous Developer Previews.
While the landing page for the beta program is live, the forum post with the download links is not. However, the download links are already floating around (OP8, OP8 Pro). You don't need to unlock the bootloader to install these images. Just begin the upgrade via the "local upgrade" option in the system updates settings, but be aware this resets the phone. Going back to a stable version will require erasing the phone (again).