In Paris, subheadings can be added to forms in Workspace. Below the documentation:
Set up form headers in workspace

The issue happens when the "Subheading" field needs to be set back to null (no field), but there is no way to do it.
This field is not mandatory, so the field should accept null. 

 

Note the "Subheading" is a "Field Name" field type with the following arguments:
allow_null=true,allow_references=true 

1. Navigate to Workspace Experience > Forms > Form Headers and click New
2. Fill the fields as follow:
Table = incident
Primary field = short_description
Subheading = Select any field. For testing, any field is ok.

3. Save the record
4. Try to set to null the Subheading field. You can select any other field, but not null.

This problem is currently under review. You can contact ServiceNow Technical Support or subscribe to this Known Error article by clicking the Subscribe button at the top right of this form to be notified when more information will become available.