The Missing Abandoned Shack is a mythical location in Grand Theft Auto V. It is marked as one of the locations for a letter scrap on Rockstar Games Social Club checklist. There is actually no shack to be found in the game.
The Abandoned Shack site is located in Tongva Hills. When visiting the site, all that can be found is a fruit stand/rest stop. It is possible that the abandoned shack was going to be similar to the Shady Cabin found in Grand Theft Auto: San Andreas. The shack was probably cut at some point during the game's development, but its location label was not changed when the Grand Theft Auto V checklist was launched on the Rockstar Games Social Club. Interestingly, the site appears to be leveled for a larger structure than a rest stop.
It is unknown why the shack was removed, or why it was never taken out of the website.