A lot of older telephone central offices still have those signs. I saw one at 10 S. Canal in Chicago a few years ago.
About 10 years ago I was working at a C.O. in suburban Chicago that still had that sign. While digging through some file cabinets looking for some drawings I found a copy of the Bell System manual for managing fallout shelters. It addressed who was in charge, how to maintain the equipment in the building in case of nuclear war, how to ration supplies, how to check the outside air for radiation and when to do so, etc. I wish I had kept it. It was a pretty interesting document.