One of the best post-modern German/Austrian novels is Die Angst des Tormanns beim Elfmeter by Peter Handke. Elf = 11. The Goalie's Anxiety at the Penalty Kick.
So I reached out to some young refs in Australia. Players specifically ask the ref for 9. Refs specifically say "Get back 9" SO...there you go. In summary, I'd say it's a mixed bag around the globe.
I ref in Australia and have never heard any player or referee speak in 9s. It is always back 10, which by most referees will be interpreted as 10 metres because no one knows how long a yard is. So in Australia the wall will probably further back than in the US. Although when marking out a field it will be done correctly in metric distances. The 6 and 18 yard boxes are still called that just as a hang over from the past.
Well seems I can't get a straight (uniform) answer on this seemingly innocuous question I posed some time back. I've got some people saying they have never heard 6 yard and 18 yard (although I grew up with it) and say "9" for distance....and now I hear the opposite. Oh well...research over. Too hard I think.