Even strict adherence to semver allows this because the major version is 0. You can do anything you want until your 1.0 release. https://semver.org/ says:

> Major version zero (0.y.z) is for initial development. Anything MAY change at any time. The public API SHOULD NOT be considered stable.

That said, the GNUnet project has never, AFAIK, said they are following semver. Semver isn't the only versioning scheme around.