Summary:
If a running Pipeline job is paused, and the pipeline plugins are downgraded back to the non-beta levels, upon a restart of Jenkins, the job will still be paused. But the Pause/Resume link is not present, and the (X) button for aborting the run has no effect.

Steps to recreate:
1. I have this Declarative Jenkinsfile in one branch of a multibranch project. It builds successfully.

2. Set the Pipeline durability setting to PERFORMANCE_OPTIMIZED via the main Manage Jenkins --> Configure page, and verify that this is the mode you'll run in by checking the actual job's "View Configuration" page.

3. Start a run, and pause it before it completes. The console log should show "Pausing"

4. Go to the Manage Plugins page, and downgrade all five of the durability beta plugins to the previous versions.

5. Restart Jenkins gracefully. I chose to use the jenkins cli command java -jar ~/bin/jenkins-cli.jar -auth admin:admin -s http://172.18.40.95:8080/ restart

6. Return to the job. It is still shown with a flashing status icon:

...and the console output is unchanged:

7. Notice that the usual "Pause/Resume" link isn't there:

8. Try to abort the job by clicking on the red (X) button. Click OK.

9. Notice that the job is not stopping.

10. I attempted to delete the job via the Jenkins CLI, and that didn't work either. I got a permissions problem back:

✔ ~/Documents/CloudBees/Work/Pipeline-Storage-Improvements/bitbucket-repos-for-testing/pipeline-perf-testcases [DeclarativeParallelsSleep|✔] 
17:13 $ java -jar ~/bin/jenkins-cli.jar -auth admin:admin -s http://172.18.40.95:8080/ delete-job pipeline-perf-testcases/DeclarativeParallelsSleep 3
3: No such job '3'
pipeline-perf-testcases/DeclarativeParallelsSleep: admin is missing the Job/Delete permission

ERROR: Error occurred while performing this command, see previous stderr output.

That's strange, because this instance has "Logged in users can do anything" set, and the admin user has no trouble doing anything else.