Mostly these are called block standing agreements and they are between sectors and areas. They are published, but as you say, they are not openly published.
This is an area where ATC and Operators really need to communicate better becasue we as operators are expected to fly min fuel, our planning department work on what they know, yet we are still expected to descend to some unexpected level, so many miles before a point.
Why can't you ATC types just organise things so we have to be level by X waypoint and publish it widely.
This is not a secret society, this is an industry that is trying it's best to save fuel.
It makes me seeth
