At the bottom of the Piccadilly Line escalators at Holborn, you turn
right to reach the platforms. As you enter the tunnel at the bottom of
the escalator, there is a large, heavy-looking, apparently steel door
set into the wall on the left-hand side of the foot tunnel.

Other than having been painted open, it looks like it is in working
order (it even has a small handle).

What was the purpose of this door, and is it still available for that
use today?