I've only ever seen it in the document that the photo in your post is taken from, never in the wild.
I've done voice reinjection myself by taking the wire that was going off to extensions and punching it down into a RJ11 keystone. It's physically disconnected from the Openreach network so no danger of any issues coming up, and I doubt this line will ever have a voice service on it again so I'm not fussed about the permanent nature of it.
The other trick (yes, I know it would be a treasonable offence etc) is to take one of these...
https://kitz.co.uk/adsl/images/phone/LJU4-1A.jpg
Use one side for the 'Exchange Facing' connection, remove the components from the other side, and wire the extension network to that.
It would look a damn sight nicer than the '5C' abomination range of sockets (designed for people that can't use a screwdriver, or a Krone Tool).
Actually in this brave new (interim) world of SOGEA, surely the only component now required in the master socket is just the Surge Protector ?