Have you ever noticed that very few newer buildings have them?
Modern building codes require egress staircases to be placed internally in a fire rated space. The exact rules will vary by building and municipality, but in the US, you will generally find that egress stairs are isolated from the main building space. These are not the fancy glass stair case leading to the mezzanine of your designer venture capital office. Egress stairs are the ugly steel stairs encased in a vertical concrete shaft with heavy metal doors at each floor. These stairs are designed to not have flammable materials, to not allow fire to spread up through the shaft, and finally the stairs and walls are designed to withstand a specific amount of heat for a specified time, for an egress stair, this is typically what is known as a 4 hour rating. So in theory, if a fire is burning outside the stair for 4 hours, the stair within should still be functional for someone who needs to escape the building. There are actually specific prescriptive tests for fire ratings of various materials, generally Architects are your best source for information on this subject.