When adding a multiplexer event to a workflow, user groups that have been moved to trash are still shown in the "Users without workflow IDs" list.

Steps to Reproduce:

1. Navigate to site backend
2. Create a new User Group
3. Remove the created User Group to Trash
4. Under Setup > Workflows, create a new workflow
5. Add a multiplexer event to it
   You will be able to find your removed user group in the "Users without workflow IDs" list.

Check the attached screenshot for example.