I've had this happen, yes.
Back when TS4 was brand new (only Willow Creek, Oasis Springs, and Newcrest), I found a larger house on the gallery that I decided to use for a 5sim family in Willow Creek. I placed the build in the Potters Splay 30x20lot, in the Courtyard Lane neighborhood. My sims family lived there very happily for a couple of RL years. Then the worst happened ... my hard drive died and I needed to replace my computer. Unfortunately, I lost some of the TS4 data from my played game (I only ever keep one save file), so once I was able to transfer the save data that had been rescued from my old hard drive, I discovered that the house and everything from when I had moved my family into it was gone. The house was even gone from my game library; I didn't panic, though, as I could remember exactly what the house looked like (and I still do, even after all this time) and figured all I needed to do was search for it on the gallery again. But, yeah ... I have never found that exact build (nor one even close to it) on the gallery again.
My building skills have improved enough over the years that I could probably rebuild a fairly close version of the house, but not sure I want to invest that much time in the project, as the family I had moved into that home has separated into different homes as the children grew and moved away, while the original parents have long since passed on. (The eldest kid from that family still lives in Oasis Springs, while her twin siblings live in San Myshuno.) Honestly, though, while I still somewhat mourn the loss of that house, I just always figured that the person who had originally created it cancelled out their TS4 account for some reason, so everything they had shared was deleted from the gallery's server.
