Need to collect SSN and then delete after ticket is closed

It would be more manual but you could send a retake link to the ticket owner and then you could hide all the questions using display logic with Q_URL, so that only the SSN question appears (less clicking around for them) and then have them delete the SSN from the survey response and submit the survey. If you would like additional details let me know. 

 If the question is a multiple-choice question or a form field, you can open the options menu in the SSN field and select the option to exclude it from analysis so that this information is not saved. 

exclude analysis

Excluding from analysis still captures the data data on the backend and does not fully remove it from the response - depending on your legal requirements you may or may not need any sort of capture of this nature.

The way we’ve done this in the past is by creating a “fake” form and storing answers in ED fields instead of using a regular form question.

Contrary to answers, ED fields values (and tickets additional data) can be manipulated via API calls and/or workflow tasks.

That way, you can have a workflow triggered on status change with update ticket and update survey response tasks to redact sensitive fields.