Just to explain the existence of its warning; with a different version numbers, the client would automatically assume that the miners could be voting for activation of an unknown rule for the network. This could be dangerous for the client if the client doesn't know what the miners are signalling for and thus what the change is about.
In this case, it's simply the fact that the miners are using overt ASICBoost which changes the versionbit to one that is non-standard. The client would automatically display a warning as it has an unknown version but it wouldn't be that big of a concern to most since it only appears in the debug.log.
Thanks, ranochigo. That's interesting, so potentially it could be dangerous, so perhaps the "warning" word is correct. So the client is throwing a warning because it's completely in the dark but everyone else knows what it is. I think I can live with that!