In Prison Architect, how do you set an area to be accessible only to staff

Via Bureaucracy, research "Deployment":

This requires a Warden - to unlock the Chief - the Chief, and offices for both.

Once unlocked, you get access to the Deployment tab in the menu bar, which offers a bunch of selections:

There, select "Staff Only", and click on the area you want to change:

This means that only staff are allowed to enter this area (in this case, the Chief's Office) on their own - Guards can still escort prisoners through these areas, and staff will not avoid secured areas, just because a staff only route is available.

As such, yes, it can be used to prevent escapes (hints from the Deployment wiki article:)

• By designating areas such as the kitchen, and the corridors leading up to it as staff only, you can greatly reduce the amount of knifes stolen. The same principle applies to storage.
• If you have fenced off your entire prison, you could designate the entire area inside and outside the fence as staff only to isolate cell blocks from eachother. Do remember to provide passage to canteens and showers if these are in a seperate building.

Also: This is not a new feature of Alpha 10. Alpha 9 introduced the new Jobs feature, but this has existed for a while before already.

it's in the deployment tab, available after you researched it in the chief's tech tree