The Crumpled Letter is a diary entry that can be collected in Metro Exodus.
Location[]
Moscow: After Artyom is shot by Hanza soldiers, you will be forced to crawl under some dead trees. Right after you reach the other side and grab the medkit, you will see a body right in front of you, half-buried in an earth mound. The crumpled letter should be on it.
Transcript[]
Seryozha, we made it!
You can drive almost to Ramenskoye, just say away from the highway, it's all blocked with scrap. From there, you'd have to walk, of course, at least across the Ring Road. This place is alive all right, the guys they have in their army are healthy as bulls! They're taking us to a filtration camp, but I don't think they'll keep us there long, a couple of days tops, then they'll find some work for us. I already heard the guards talking of being short on hands! I say, you guys drop everything and come over here, I'll get everything ready!
I'll send this letter as soon as I can.
Dad