Most of my experience with it has been dependencies jumping onto new features once they stabilize. These tend to happen in waves as larger features land. Some of the latest points I've seen:

- 2018 edition
- byte accesses on primitive types (to_le(), as_bytes(), etc.)
- code changes to take advantage of NLL
- async/await
- feature(doctest)

These usually have 3-4 releases between them.

If you want to avoid these kinds of things, keep a Cargo.lock and you'll stick with versions that continue to work. Very rarely things do get removed in Rust updates, but these are typically soundness issues that can cause problems if they are used improperly. Luckily crater can be used to fix it across the (FOSS) ecosystem pretty easily. Release notes are well-written and short enough that anyone else can read and follow along.