Team,
Currently we don't have a way to enforce OPA policy only on newly created pipeline. When trying to enforce the policy using "on Save", this gone a impact even the existing pipelines. As there is no pipeline creation time available from YAML, its hard to differentiate new vs old from a particular day of this policy implementation.
FYI
As suggested by support team (https://support.harness.io/hc/en-us/requests/98558) raising this feature request and are requesting to have prioritize.