Input validation is performed to ensure only properly formed data is entering the workflow in an information system, preventing malformed data from persisting in the database and triggering malfunction of various downstream components. The checks on inputs openshift-router perform are not on par with Kubernetes checks for ports, Service names (and additional backend service names). Indeed, the invalid format for those is implicitly detected when ensuring the objects exist in Kubernetes before producing the Haproxy configuration. Relying on such implicit validation is dangerous as it exposes a wider attack surface to invalid inputs.
One way to abuse this lack of explicit validation is to reflect those in the oc command line. When inserting terminal escape sequences, it is indeed possible to manipulate the entire output as viewed by the user. As such, this issue can be considered a new vector for an already known attack: CVE-2021-25743.