This is the 5324th most frequent Turkish word.
kaybolduğunu
"The fact that [someone/something] is lost."
In this sentence, 'kaybolduğunu' is used to indicate the idea that an item (the wallet) was perceived as lost.
Cüzdanını bulamayınca kaybolduğunu düşündü.
When he couldn't find his wallet, he thought it was lost.
Here, 'kaybolduğunu' expresses the concept of the loss of something intangible (childhood) being felt or observed.
Bana çocukluğun kaybolduğunu hissediyorum dedi.
He told me that he feels childhood is lost.
In this sentence, 'kaybolduğunu' communicates the author's focus on the disappearance of intangible cultural practices.