All those things happened when the Rust crate ecosystem was still much in flux (back in 2017), and I had some good reasons:
- Serde had made a very slight but breaking change in 1.0, and at that time I think it was impossible to support both Serde 0.9 and 1.0 in a single crate without a hacky workaround (which I only learned much later). So if I had to pick only one version to support, it ought to be 1.0 as the change was trivial to resolve.
- Cargo's use of semantic versioning is, while documented, not strictly conforming because 0.x.y is considered compatible with 0.x.z where x > 0 and y < z [1].
- People complained a lot when Chrono went 0.2.x to 0.3.0 as well. This is IMO the biggest reason to issue a breaking change; if people would complain in either way, I wanted to make a choice that benefits the whole Rust ecosystem more.
If this happen today I would agree that I shouldn't have done that, but I think it was not that clear cut at that time.
[1] https://semver.org/#spec-item-4
