NLnet. However they only fund specific types of projects, and there are many open source maintainers who are not interested in money (usually they have a well-paid job already).

I havent noticed any problems with instability, at least for web server development it is stable enough. But it may be different in other contexts like embedded. And its true that many libraries still have 0.x versions.

??? Rust 1.0 was released 10 years ago and since then there have been no breaking changes.

https://blog.rust-lang.org/2015/05/15/Rust-1.0.html