I installed the 'Maven Dependency Update Trigger' plugin, and restarted Jenkins.

Then I configured an existing job, under 'Build Triggers' -> 'Maven Dependency Update Trigger' (checked) -> Schedule: */15 * * * *

I also check the checkbox 'Check Snapshot Plugins Update' and save the changes to the Job.
The next time I view the config for this job, the 'Check Snapshot Plugins Update' checkbox appears unchecked.
This makes it unclear if the settings is active or not.