While the evidence seems strong at first blush, it's not certain that this is burn-in. There's a chance this is image retention (something more commonly associated with LCDs), which could go away on its own if you introduce some variety to what's happening on screen. It's also uncertain just how many people are affected. There are certainly other reports of burn-in, but there's a difference between anecdotal stories on forums and widespread issues.

Whatever the case, this definitely isn't what Google wanted to deal with. There are already other complaints about the Pixel 2 XL's display, whether it's colors that aren't as vibrant as on some phones (albeit more accurate) or a blue tint when you view the screen from a sufficiently wide angle. The Pixel 2 XL is supposed to be one of Android Oreo's two showcase devices, but it may end up leaving a sour taste in people's mouths if it's more commonly associated with display flaws.