Keep clear of controlled airspace!
This is being uttered to aircraft not intending to join CA by at least 50% of ATCOs these days so presumably it is a "new thing" to say. It is usually a totally useless piece of advice and it is another sign of dumbing down and saying so many things that it is getting increasingly difficult to respond to everything that is said without asking for a repeat. Net result: too much talking.
If it is being required because the CAA actually believe that it will help stop pilots from wandering into CA without a clearance, I can think of many other things to tell a pilot so that he is aware of some hazard or other. Where do you draw the line?
Another bad thing about it is that it is being uttered by rote. Just another thing ATCOs always say. Something that eventually you do not hear, or heed. ATCOs are being drilled so much they are becoming robotic, and eventually they will be replaced by computers. Frankly, it's a bit pathetic.