Summary:
After taking the supplied zip of scm-api plugins, and applying it over top of a previously known good Jenkins 2.42 instance, the warning "You have data stored in an older format and/or unreadable data" appears.

Steps to reproduce:

1. Set up Jenkins with a handful of Bitbucket and GitHub multibranch and organization jobs.
2. Stop the Jenkins service entirely.
3. Unzip the supplied scm-2.0.x-plugins.zip file on top of the /var/llib/jenkins/plugins directory
4. Restart the Jenkins instance
5. Sign on again

Results:

I will upload logs and everything else soon after getting this filed.