I recall from my theory test that this was a blue line but are they a common or outdated feature nowadays?. I’ve only driven about four different artics but none of them had this line fitted.
I would say the blue line has been virtually done away with. Its been a long time since I’ve seen a Unit with a blue line fitted to it, but, for some strange reason there are still a lot of trailers with the blue line connection on them, pretty silly cos no unit can connect to it and if it did I can’t see what use it would be.
Maybe some specialist hauliers still use them for some kind of air operated equipment, on the trailer.
No they have become almost obselete on modern trucks. as usual the UK was the last to have them. Not many foreign trucks had them in the first place, especially over the last 25 years.
It was a blue line with a larger C type coupling that was linked to the trailer brake handle in the cab.
It was probably the trailer hire companies that got them removed as you could save your unit brakes and use the ones you were not paying for 
I’ve pulled a number of trailers with them (including the one I used for my test). Never had a unit with an aux connection though.