Pokestops can get removed at any time if an Ingress player reports them for a reason such as being on kindergarten/primary school grounds, not having safe pedestrian access, the POI not existing anymore etc. The stop could also have been moved, have you checked for that?
We had a gym on the Arch of Triumph in Bucharest, and it's gone for some time. I guess "not having safe pedestrian access" was the reason there.
And then there's the Orange Bicycle gym... the item the gym refers to has disappeared. I wonder if that gym will remain now.
There is a risk that the gym disappears if a player reports it as not existing anymore. If no one reports it, it'll stay indefinitely, it's not like Niantic can automatically detect that the item isn't there anymore.