What's the safeguard called, which I marked with a red arrow in the picture underneath?
Anyway, why didn't ships in the past have them, to safeguard their crow's nest lookouts? The picture on the right is the RMS Titanic, and there doesn't look like one?
Obviously, these ladders are dangerous as someone can fall, especially if a gust blows them off.
The Crow’s nest, accessed by climbing up a ladder inside the mast, had a large bell, used to warn the bridge of dangerous ahead, and a telephone so that the officers on bridge could communicate with the lookouts in the crow’s nest.