We've received a few reports of the update so far, and our own Artem has received the update on his Pixel XL as well. As you can see from the below screenshot, this was clearly not meant for public release:
Being just a security update, there likely isn't any user-visible changes. One tipster said he received the new Play Store UI after updating, so maybe it triggers (either accidentally or on purpose) some server-side changes. Let us know in the comments below if you see it on your phone.