Try installing this plugin:
https://wordpress.org/plugins/wp-crontrol/
Then check the Tools>Cron Events page to see if the “pta_sus_cron_job” hook is listed there. If not, deactivate and reactivate the pta sus plugin, which should add the cron job again.
Please note that WordPress CRON jobs are not true CRON jobs, as they rely on traffic to your site to trigger the CRON checks to see if anything is due to be run. If you don’t get any site traffic for a long period of time, the jobs will never be triggered.
Unchecking the automatically clear expired signups will not affect the scheduled cron job. The cron job my plugin adds will simply skip the check for expired signups if you uncheck that box, but it will still check if it needs to send reminders.