CRDs allow unrecognized formats to be written into OpenAPI schemas. This conforms with the JSON Schema specification, which states that unrecognized formats should be allowed, but ignored, by implementations.

This has a few consequences:

• Typos in format names are not caught by CRD validation. Instead the format is ignored and CRs can be written that are invalid according to the format.
• When Kubernetes introduces support for new formats (example), it is possible for Kubernetes versions that do not recognize the format, but the format is ignored and CRs can be written that are invalid according to the format.

Adding a warning when unrecognized formats are written to CRDs can help communicate this when CRDs are written.