This is the first time I've heard of this. It's also unenforceable, since it would require someone at the top of the stairwell, which is some distance from the desks (and in B and C, out of sight of the desks). I can't see anything on the HAL side. The escalators break down or go out of service from time to time, as all escalator can do. As for incidents, the one fatality I know of at T5 was on the escalators outside GC North, which explains the permanent HAL presence there. That was the escalators, not the stairs.

That said, the usage of stairs is a major form of workplace accidents, probably the biggest one in office based workplaces.