I've been working on a patch for ws-cleanup-plugin to make it possible to use "Clean workspace before build" in a (declarative) pipeline. Apologies if I have misunderstood things, but I haven't done that much development of Jenkins Plugins before.

The code in question was a BuildWrapper, but is now SimpleBuildWrapper, to make it compatible with declarative pipelines.
https://github.com/jenkinsci/ws-cleanup-plugin/pull/49

When used in a non-pipeline job the wrapper is executed during the preCheckout phase IIUC. That means that it is executed before the job checks out code from SCM. When used in a pipeline job the wrapper is executed after the default checkout from SCM. This can be seen in the logs.

It took a while for me to understand that the method runPreCheckout() is only being evaluated for non-pipeline builds. Is there a technical reason for this? If not, would it be possible to change this so that runPreCheckout() is evaluated for pipeline builds as well?

I have managed to work around this by modifying the declarative pipeline to look like this:

pipeline { 
    agent any
    options {
        skipDefaultCheckout(true)
    }
    stages {
        stage('Build') {
            steps {
                preBuildCleanWs {
                    checkout scm
                    echo "Building ${env.JOB_NAME}..."
                }
            }
        }
    }
}

That however doesn't feel intuitive and, as I said in the beginning, this plugin behaves differently in pipeline and non-pipeline jobs by default.