Removing `non_exhaustive` can potentially be a breaking change

[ehuss]

Per the discussion in https://github.com/rust-lang/reference/pull/1837/changes#r2532838829 and rust-lang/rust#147722, there are some edge cases where removing the #[non_exhaustive] attribute can potentially be a breaking change. That issue has a proposal that could fix that issue, but I wanted to create an issue here just to track this just in case it doesn't get resolved. If it doesn't get resolved, we may want to consider adding something to the SemVer compatibility chapter.