I'm not sure exactly what I'm looking at here, but this is coming up in the console in Chrome on multiple sites. I do not see it here on XenForo.com. This is using the default, unmodified style with zero add-ons enabled.
Well it will be awhile, but judging how quickly Chrome 89 is out, the developers probably should know about this. F12 -> Application from the PWA and you will get the warning. At some point, when Chrome 93 hits, the PWA will probably stop working. Improving Progressive Web App offline...
xenforo.com
It can be disregarded for now (the change don't actually apply until Chrome 93).