O365 Groups and its related content (Plan, Files, etc.) when Group Owner leaves the organization?

There is no way to recover SharePoint data once the group has been deleted.  If you want to recover files for groups without owners you will need to add a user as an owner of the group to access/move/copy the files.  

Here is a script from the Office 365 for IT Pro book to find out which groups do not have owners:

# Find out which groups do not have owners
$groups = Get-UnifiedGroup
ForEach ($G in $Groups) {     
    If ($G.ManagedBy -Ne $Null)      
    {          
        $GoodGroups = $GoodGroups + 1     
    }     
    Else     
    {             
         Write-Host "Warning! The" $G.DisplayName "has no owners"          
         $BadGroups = $BadGroups + 1      
        }
    }Write-Host $GoodGroups "groups are OK but" $BadGroups "groups lack owners"