see https://github.com/CycloneDX/specification/releases/tag/1.6
and https://github.com/CycloneDX/specification/releases/tag/1.6.1

The following items are not currently supported for CycloneDX v1.6 (as of release 7.6.x):

• definitions
  • partially addressed by feat: add basic support for Definitions #701
  • partially addressed by feat: add extended support for Definitions #713
  • TODO: definition.Standard.signature
• dependencies.provides
  • via feat: add cyclonedx.model.dependency.Dependency.provides #735
• bom-ref properties here and there
  • issue model.contact.OrganizationalEntity missing bom_ref attribute #799
  • possible fix: feat: add BomRefs for known models according to CycloneDX 1.5 #859
• declarations
• to be continued

this library is a community effort.
if you find a feature that you need is missing, feel free to donate/contribute the missing feature.

• see 📣 Call for contribution #633