~100k is a thing on Linux.

On Linux, 400k threads was a thing on even on 2002 hardware that was 32-bit and thus limited to 4GB: https://news.ycombinator.com/item?id=37621887

In the titular post there's a link to a previous comparison between approaches, and plain OS threads used from Rust fare quite well, even if the author doesn't up the OS limits to keep that in the running for the higher thread cases: https://pkolaczk.github.io/memory-consumption-of-async/