Jenkins Freestyle Project.
Running a post-build step to upload files to S3 with the s3-plugin version 0.11.5.
Specify S3 source, destination, profile. Standard usage of the plugin.
About 6000 files are uploaded.
In the jenkins log /var/log/jenkins/jenkins.log are about 6000 warning messages:
2020-05-07 21:04:54.461+0000 [id=801] WARNING hudson.util.Secret#toString: Use of toString() on hudson.util.Secret from java.lang.String.valueOf(String.java:2994). Prefer getPlainText() or getEncryptedValue() depending your needs. see https://jenkins.io/redirect/hudson.util.Secret/
The text of that warning is apparently found in jenkins/core/src/main/java/hudson/util/Secret.java
As far as I am aware, the s3-plugin is being used in the standard way. Nothing has been customized. Just uploading files to S3 after the build finishes. Glad to provide more information if that's helpful. This problem has been reproduced on two different Jenkins servers, so it seems consistent. You may try.
The goal would be to solve the error, or at least not spam the log file with thousands of warnings.