This change happened in a patch release, so the serde author either did not consider it breaking, or doesn't follow semver in the first place.
Most crates do not raise their major version when increasing the minimum supported rust version. So there is precedence for treating which changes can break for some people, but which don't have a breaking API changes, as non-breaking for semver purposes.
Most crates do not raise their major version when increasing the minimum supported rust version. So there is precedence for treating which changes can break for some people, but which don't have a breaking API changes, as non-breaking for semver purposes.