Pipeline libraries now come from external sources in Jenkins, but it is not convenient for static images due to performance overheads. Especially for single-shot masters you do not want to download libraries each time

https://plugins.jenkins.io/filesystem_scm allows providing Pipeline libraries from the filesystem, and https://github.com/oleg-nenashev/demo-jenkins-config-as-code demonstrates usage of such flow for local development.

It might be useful to natively support this mode in configuration YAML files.