How do you create a workflow on a related-list record unassignment event?

Hi all,

When a user unassigns a related list record, I need to update the main record, but I can’t get any trigger for that, meaning the system doesn’t notify when it happens so I can’t run any workflow.

This is a pretty painful issue, and I am curious how do you handle this. Are there any workarounds? Do you prevent unassigning and use a different approach?

I would be happy for you insights