Tool author here. Your hunch is absolutely right. An analysis I and a few contributors did for Rust showed that over 1 in 6 of the top 1000 most downloaded Rust crates has broken semver at least once in a way our tool can detect & where we can produce a program that suffers a compile error due to the semver break.

This is obviously a lower bound, and yet it's quite a high number already.

I plan to write more about this on my blog in the coming weeks:

https://predr.ag/blog

In the meantime I put some more info (including a link to slides from a recent talk I gave on this) in another comment:

https://news.ycombinator.com/item?id=36653954