Hi,

Why use physical deletion of record when ServiceNow provides logical deletion (Active: False/True)

To answer your question, if Assignment Group is deleted & if was used (not necessarily for incident, changes, etc. can be any record type) all it will show up will be sys_id that might be difficult to interpret & understand. As you are aware sys_id is unique 32 bit identifier & is difficult to understand. 

For instance, you can try creating a group in PDI associate it to a record & then delete the group so as to understand what happens with the existing data that is using the assignment group record.