The shelters that have remained in good shape are almost all those with walk-in entrances and secondary uses. The ones with same-level entrances from basements or into the side of a hill, and particularly those with wide and easy to use doors are far more likely to get maintenance and remain in serviceable condition. Those that you have to open a bulky hatchway, climb down steep and narrow stair, worse still climbing down a ladder, tend to get ignored and neglected.

The divide is between those where you can open the entry door with one hand and get into while carrying a box in your arms, and those you can't. If a shelter meets that standard you can use it for many other uses like storage, playroom, wine room, or pantry. If it gets used; it gets maintained.

many shelters were built 60s, there was a big uptick in shelter building around the time of the Cuban missile crisis.

As close as it was we actually got closer to a nuclear exchange than people at the time realized. Recently it has come to light that a Russian sub captain had nuclear weapons and both the authorization and mission defined reasons. Thankfully he declined.

His account is that even though he had reason to use them, and depending on how you interpret his orders, the duty to, he didn't want to be the man who started WW3. Like so many other events in history it was down to the conscience of a single fairly unremarkable man that made the difference. It was the conscience of a captain in the navy of serving the organization we would later brand as "The evil empire" that saved us all. Point being we need to be careful about discounting the humanity of, and vilifying, our present-day enemies.