The canonical example is, I think, the aborigines of Tasmania after they were cut off from the mainland. Over the 8000 years of isolation, the ~10,000 Tasmanians lost most of their technology - fishing, the ability to make fire or bone tools, etc.

The story, as I read when visiting relevant sites in Tasmania (and confirmed on wikipedia) is one of "necessity is the mother of invention" in reverse - Tasmania being a veritable paradise compared to most of the mainland. Fishing traps for scaled fish were not especially important given the effort they required compared to eating shellfish and seals while environmental changes and increasing tribal territory sizes made hunting land animals even easier. They also never lost fire.

This example isn't especially strong evidence that technology loss is caused by a 10k population size.