I don't know about your system, but in ours (Canadian), the information is often not available, simply because there is no formal system/infrastructure for providing it on anything other than a case-by-case request basis.
Our agency oversees federal staffing and attempts to gather such information by means of manager surveys. We ask them how many applicants there were at the outset, how many qualified people they had at the end, how many positions they were trying to fill, and whether they filled all, some, or none of the positions. That's more than we used to gather, but that's about as detailed as it gets (too many other things to ask about!). And believe me, there are a lotta round numbers provided by managers when it comes to how many they started out with. I suspect this is probably because the precise information rests with the HR advisor/s they worked with, and the manager has only a vague recollection. Our approach is to contact managers directly about a single process, via a "cold call", as opposed to getting HR advisors to hand over data on all the processes they handle. Obviously, we forfeit certain types of precision or information in consulting the one group vs the other, but that's how we do it.
One of the things we have noticed in our data is that when the number of qualified candidates to positions available gets higher (i.e., the ratio), managers tell us they are more likely to turn to things beyond the stated essential requirements to help make their pick. Which makes perfect sense. If you have exactly as many qualified candidates on your short list as you have jobs, then you don't have to consider anything beyond what all the testing for essentials dictates. When you have to decide between people, though, you want a "tie-breaker".
Certainly one of the things we have also learned is that what managers tell us they are selecting for is different than what candidates think they are being selected or rejected for. That gap in understanding concerns me, because it is a breeding ground for perceptions of unfairness. When candidates think that A is more important than B, C, or D, and believe they were rejected because of A, when really the manager placed more emphasis on D, that easily generates jaded views of assessment, and perceptions of rigged selection.
"Transparency" is a construct that I think senior management and officials rarely think through comprehensively. It can mean a lot of different things to a lot of different people, at a lot of different points in the process.