This feature is rude and unwelcoming.
You're not singling out behaviors that need to change. You're dropping a blanket accusation of poor behavior on everyone. The very idea that everyone needs this warning is implicitly accusatory.
Even worse, this change violates one of SO's core tenets: that it's the content that matters, not who writes it. The newness of the user is completely irrelevant. Either the user made a quality post and followed our rules and norms or they didn't. The result should be the same either way: downvote, closure, deletion if bad, upvote if good, or no action if not good enough to warrant an upvote. Possibly a comment explaining the problems or requesting improvements. This feature sends the message to all users that "new" users should be treated differently, and they should not. They should be held to the same standard. Period.
Because of this, it also means you're attaching a new stigma to new users that SO has fairly successfully avoided since its founding. You're implying that new users are incapable of writing good posts. This only undermines your own efforts to be welcoming. I have never subscribed to the notion that our problem questions are primarily due to users being new or unfamiliar with the site; the primary cause behind bad posts is a lack of effort. Bad posts come from an unwillingness to spend the time required to obtain some level of understanding about the nature of the problem they're facing or to clearly present that understanding. This is completely unrelated to the newness of the user. This new feature instead wrongly implies a causation of newness, rather than the mere possible correlation. Our moderation efforts tend to drive users unwilling to spend that effort away, at least somewhat, so it's not surprising that we'd see a correlation without a causation.