I find this tends to stem from libraries refusing to declare 1.0 for fear it would lock them into bad decisions, not from being unstable. Chrono is a great example: v0.4 for EIGHT YEARS while they make sure the design and APIs are worthy of being set into stone as 1.0 (think: Stability Bit Versioning).

Sure, some pre-1.0 libraries in Rust land are actually wildly volatile, but I find that's not especially the norm, out of the crates I've used. That said... 0.4 for EIGHT YEARS is also a pretty darn good sign you've solidified the API by now, and should probably just tag a 1.0 finally...