Prior to the Moodle 3.4 upgrade and the unified participants/enrolled users page there was an option to filter by 'no group'.

Within the new participants page the drop down menu shows all group names etc but the option to filter by no group is no longer there.

This was a useful feature which we would appreciate being returned. 

This issue is in addition to the existing issue relating to the filter being wiped out when 'paging' through large lists of enrolled users.