The Credentials plugin has provided support for IDCredentials to identify a credential for many years.

Supporting non IDCredential based credentials adds overhead to the credential plugin and this plugin is one of the last that are still known to use the legacy behaviour (which may become unsupported in the future).

Please update https://github.com/jenkinsci/loadfocus-jmeter-load-testing-integration-plugin/blob/d2d2af5b0af276932dfb6c8191d3bb13791d6bc7/src/main/java/com/loadfocus/jenkins/LoadCredential.java#L13 and any other Credentials to become IDCredentials and update the code accordingly.