With the latest snapshot, the sidebar display slot no longer renders, regardless of what objective you have assigned to it.
Steps to Reproduce:
1. Create an objective of your choosing. For example, "/scoreboard objectives add test dummy".
2. Set this objective to display in the sidebar. With the above example, "/scoreboard objectives setdisplay sidebar test".
3. Give yourself a score in this objective. With the above example, "/scoreboard players set @p dummy 10".
4. Observe that, despite the fact that you have points in the objective, no sidebar renders on-screen.
It's worth noting that scores are tracked; they just won't display on-screen.